are you making sure that the throttle is opened when carrying out the compression test.

you will just have to open your engine up to see if it needs a new piston.

mc21 engine will not bolt into an mc16 and all electrics are different.

the engine was probably running very lean it should have been upjetted for the airbox mod and pipes.

if the engine was rebuilt it would not be good to run it in on such a long journey.

just get top end done and make sure oil pump is working and get the bike jetted accordingly.