Try the Barnett purple pressure plate. If that doesn't help I'd go too the 6 finger centrifical lock up system. You can run 3 fingers for mild or 6 for full lock up. No more problems. Let me know if you would like to do this and I could get you the address of the guy in FLORIDA that does it
I would try to seal the tank with the white coating sealant. Might take care of any small holes in tank. Then I'd remove and check all hoses too see if they are leaking. See what that gets you
I would run some cheap 30 w from dollar store to flush it out more. Then take clutch apart and clean all clutch disc. Don't want milky oil on them. That is why clutch is slipping
I know your daughter would what you to finish this bike rebuild. It would make her proud, but more so, I think it would have more of an effort on you. Please show pictures when it is finished. I think this will be good for you.