I am hoping for a bit of advice before I get the spanners out

My lads Beta 50 has started jumping out of gear, he says 1st seems OK but it has jumped out of 2nd 3rd & 4th, don't know that when it jumps out of 3rd or 4th it finds some sort of "false neutral"

I have looked at the exploded views on the Beta website and there does not seen to be much to go wrong, which bits are likely to be worn?

He has had a nasty experience when the bike jumped out of 2nd on a big (for him) climb, need to build his confidence again.

Thanks

Bump! (sorry)

Fair enough nobody knows a likely cause or the fix , I don't mind stripping the thing down and replacing any suspect parts, so my next question is where can I get a workshop manual? either printed or on-line, thinking about torque settings, refill of cooling system etc.

Thanks in advance