There are several ways to approach this, and there are two 'problems' that you are looking at.

The first is the panoramic format...

One could use the approach done by photographers who shot APS-P images (or put a mask over a 5x7" (127mm x 177mm) slide or negative to get a a 60mm x 170mm frame (example: Above it all - the Arca Swiss 8x10 is an 8x10 camera... though one could put a roll film back on it, but then thats just doing it in camera rather than with a mask in post production - all the photos in that album are done on a 4x5 or 8x10 camera). There is noting saying you can't just crop a 36mp (2:3 aspect ratio) image down to a panoramic 24mp image (1:3 aspect ratio).

The other approach to deal with the panoramic format is to stitch images together. There is a large number of approaches to this depending on how much fidelity in the image you are after. You could go for something like Ted Orland's holga panoramas which have an overall panoramic aspect ratio to them.

Given that you are after something that looks more like one image, there are two options. One is with a lens, the other with the apparatus to take the photographs precisely.

Some camera systems have a tilt shift lens. You are after the shift (not the tilt). For example, the Schneider PC Super-Angulon 28mm F2.8, or the Canon TS-E 17mm f/4L Tilt-Shift Lens or the Nikon 24mm f/3.5D ED PC-E. Those are just a few of them, there are many others out there. You will find those prices a bit on the high side, but you are looking at very specialized lenses. More about this technique can be read at Why would I want to use a tilt shift lens for panoramic photography?

A large format system can get twisted any which way you want. The large format photographer would take one photograph, with the rear standard shifted to one side, and then another with the front standard shifted to the other side. The two could then be overlaid and produce a panorama that is larger than what either image could do alone. More on the shift technique at Shift panos – using shift lenses to create mini panos and Photo Technique: Tilt-shift Lenses. The key is to not move the lens and to have a lens that has an image circle that is larger than the film (this is the case with PC/TS lenses). Each frame is just part of this larger, single, projected image. With a tilt shift lens, you will need to remember that after shifting the lens in one direction, you will need to move the camera system the same distance in the other direction to keep the lens in the same location between shots (this is most easily done with a panoramic head).

The other approach is to precisely pivot the camera to allow it to to capture overlapping images with minimal distortion. This involves identifying the nodal point for the lens (each lens is different) and then pivoting the lens around that point. This often involves working with a panoramic tripod head.

On the wide shot

The XPan had three lenses, a normal lens (90mm), a wide lens (45mm), and an ultra wide lens (30mm) that came with its own special viewfinder. The horizontal field of view of each of theses on a full frame camera was about 15mm, 24mm, and 50mm.

You don't need the fisheye to get close to the 15mm field of view. A good 16-?? or 17-?? zoom lens will do it for you quite nicely (and cover that 24mm range too). Defishing the image is difficult, and taking it to the panoramic aspect ratio will throw away more data than just cropping the image on a rectilinear lens. If you really want those extra degrees, there are some good 14mm prime lenses such as the Canon 14mm f/2.8 or the Nikon 14mm f/2.8. Again, these are not cheap lenses.

If you are doing the stitched approach, you are more likely looking at a 35mm or 50mm stitched in portrait orientation, and you can have as many frames as you want there to get whatever horizontal field of view you want. This also gives you lots of data to work with and allows you to crop vertically or correct the lines (and throw out data in other places) while still providing enough data to make a good photograph.

The true advantage of the XPan is that it was one shot (rather than stitched) allowing you to have those ripples in the water (or waves) be consistent. It was also a rectilinear lens meaning no distortion needed to be corrected. This came at the expense of an oversized image circle (ND center grads necessary) and expensive lenses. Other film panoramic cameras with other approaches (Noblex, Horizon, Round shot) used a slit with a rotating lens that didn't capture the entire scene in one go (I saw some very interesting photographs from a noblex with a thunder storm where different parts of the cloud light up with as the camera moved across that part of the scene) and also could introduce a cigar distortion (related to fisheye perspective - note the second image in Photo Technique: Tilt-shift Lenses).

My suggestion? If you have the megapixels, 14mm lens and crop. If you don't have the megapixels to crop, get an appropriate wide shift lens and stitch the two images. Stitched panoramas from a rotated camera system will annoy you with the bent straight lines when you look at what the xpan could do.

I'll also point out that most XPan photographers, no matter how much they drooled over the 30mm didn't find it a useful lens for its price point (it was about $3k) - the 45mm and 90mm lenses worked wonderfully for most situations. The 45mm (28mm equiv horizontal field of view) was the one most people used day to day. And while I quoted expensive numbers for the 14mm f/2.8 above, the 28mm f/2.8 is a very affordable lens (Nikon 28mm f/2.8, Canon 28mm f/2.8). With this lens you will certainly be able to take the wide photos without having to go too deep in the pocket book.

And a shot of my own with the 45mm lens: