In practice, converting a film SLR like the Canon EOS 300 into a true digital camera is not a realistic DIY modification. A digital conversion would require far more than just adding a sensor: you’d need precise sensor placement at the film plane, power, image processing, storage, controls, and integration with the camera body.

The one relevant idea mentioned by the community is a retrofit “digital film” style insert, such as the re35 concept, which is designed to fit where 35mm film would normally go. That’s the closest thing to a digital retrofit for a film camera body.

So the short answer is: a full sensor-and-electronics conversion is generally impractical, but specialized retrofit products are the type of solution to look for if you want to keep using the camera body.