Valve lifter not working + worn valves/rocker ?

[ajscomboman]

You cannot get 350cc L/W pistons new from anywhere. You can however modify 650 twin pistons to suit easy option is the 59 only type as all you have to do is alter the skirt to clear the flywheel. Gives slightly more compression as well.

[ChrisTheChippy]

My guess is someone's had a timing tool down the plug hole , forgot about it and kicked the engine over jamming the piston.

[SPRIDDLER]

My guess is someone's had a timing tool down the plug hole , forgot about it and kicked the engine over jamming the piston.

That's a thought, Chris.
I've not seen a L/W cyl head in the flesh but it seems that the zorst valve seat has been replaced, perhaps as a result of damage by a piece of valve or the original seat coming adrift. Although it's quite normal for the zorst valve to be smaller than the inlet the replacement seat does seem to be a rather small diameter. Maybe it's just an optical illusion from the photo but perhaps someone (Spares dep't?) can give you the diameter of the standard valve head as a replacement valve could be too large.
I'm not sure what I'd do about the piston but I'd certainly have a good look at the inside to see if there's any stress/cracking visible.
The gudgeon pin should just about slide through the piston under its own weight. If it is really loose I'd probably go for Rob's suggestion of a 650 twin piston and also replace the little end bush in the conrod.

[Amb3611]

You cannot get 350cc L/W pistons new from anywhere. You can however modify 650 twin pistons to suit easy option is the 59 only type as all you have to do is alter the skirt to clear the flywheel. Gives slightly more compression as well.

Thanks for that I’m starting to find your right. No stock anywhere I can find at the moment Any idea what the part number would be for 650 twin piston? Also how much would need to be removed from skirt to fit the 350?

[Amb3611]

Rebuild finally complete , new valves ,guides and rockers installed, surprisingly the decomposition over now works so was possibly due to excess rocker and valve wear ? runs nicely on first test run but now appears to have electrical issues