Most of the dust visible in your example image is far too sharply in focus to be inside the actual viewfinder assembly made up of the prism and eyepiece lenses. It's almost certainly on the focusing screen at the top of your camera's lightbox. You can probably blow it off with a hand operated bulb blower. It may take several cleanings over time to get most of the dust out of the camera's light box.

Focusing screens have a micro-textured surface and are very fragile! You should avoid cleaning it by any method that requires a cleaning instrument or material to come into physical contact with the surface of the focusing screen.

Some cameras have interchangeable focusing screens that can be removed/replaced via the throat of the camera's mount (the hole in the front when there's no lens attached). Some are considered user serviceable and some are not.

Minolta offered eight different optional matte focusing screens for the X-700, but advised users to have them changed by a factory authorized service facility, such as a Minolta authorized repair shop. It's not clear whether the swap could be done via the camera's lens mount throat or if the top cover needed to come off the camera.

From the Minolta X-700 Instruction Manual:

According to this old forum post, it's a fairly straightforward but delicate process that can be done through the opening in the front of the camera. I'm not sure I'd recommend it if you've never done a focusing screen change before, but any competent camera repair shop should be able to remove it, blow it off, and reinstall it or, better yet, replace it with a new screen (supplied by you or them). Various screens for old film cameras are often listed for sale on eBay for $10-30. This current listing offers two standard X-700 screens for $12.95 plus $3.95 shipping.