How hard will it be to mount it on a regular 26" mountain bike and still be able to use the pedals?
Most bikes will have plenty of room to mount the engine, but very few will the peddles clear the exhaust pipe..this means that the pipe will need to be bent. Not a hard process.
And how hard is it to take one off and transfer it to another bike after it has already been installed?
Be about the same as installing one on the first bike. Keep in mind that the frames will be different and the engine mounting won't be the same. Keep in mind that the optimum
"V" in the frame is 75 degrees.
What you see (above) is the instillation with factory mounts. Should (as others have) need to lower the engine, a custom rear mount can be made so the engine can sit lower in the frame.
Engine Mounting pictures by Ron-Becker - Photobucket