I was just wondering what would be the advice form experts, first why, what should I look for?
Also what to do now, is it worth fixing or just keep the motor for spares? (I did buy another motor already)

Bike is PGM2 MC18, bought it as is from the guy who was trying to build a race bike and gave up, felt like it had compression but once I tested it was 130 psi front and 70 psi rear, this is why

If the piston looks similar on the inlet side it's 100% due to lack of lubrication.
There should be similar signs of wear across the boost port though not that bad as on the exhaust side.

If not, you should check for air leaks e.g magneto side seal, carb manifolds etc.
There seems like there is a crack on the exhaust bridge, check it out as well.
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2t-racelab.com

Yeah, I think bridge is cracked, is it junk now? How much is the used cylinder?

all the damage is on the exhaust side, so I thought it was overheating, but there have been many hands in this bike, not sure if it just ran out of coolant or ran lean.

TPS is completely missing, could that have caused the lean condition?

double check your jetting etc but it can happen if the bike ran out of petrol once before and ran lean on that side as the LHS carb is the furtherest away from the fuel line .
Just double check everything as you go along and ask questions .