I agree that building the engine with linear alternator on the piston is beneficial. After all iff the energy is pulled diretly from the piston, thebearings and other parts recieve less force, thus limit friction losses. This also makes for a longer life, and probably more reliable engine.
Incidentally we experimenting with a build that is really suit this design. It has free piston operating principle, can be sealed and in the current small engines it works with membranes. (or bellows for that matter) Has no bearings, or flywheel, it is timed by pneumatics and mass. The work piston needs to be heavy, so including a permanent magnet in the piston is no problem.
For the other things you mentioned:
-Rpm is important for the efficiency of the generator, or if someone uses a of the shelf generator. Hp is usually the most important factor, since it sets the limit of maximum electricity output, no matter the rpm.
-Using magnets as return springs have been used for modells. And this looks real nice. But problematic for higher power engines, a repelling magnet makes a lot of force that is other than the axis of motion. Meaning more friction loss on the guides. Most strong magnets dont like high temperatures, so they cant be used in the hot parts of the engine. But the observation is right.
If you are interested, you can check out the demonstration model that a forum mate built. It is mostly transparent, so we can better understand the workings.
slow motion, while the engine is running
running in real time
You can find out more about the design if you watch some of the other videos he made.