PGO Big Max 50 - sudden power drop

I acquired an old PGO Big Max 50 that had been badly neglected. The engine lacked power and I decided to do a bit of an upgrade. I popped the engine out, overhauled the transmission side, with new variator weights, belt and clutch. I replaced both perished crank oil seals (thanks to forum members for sizing) and fitted a 72cc cylinder kit and 19mm TNT 'racing' carb (see separate post). During the stripdown I noticed the stator side crankcase had been welded, but the repair looked good. The engine started fine, I warmed it up and adjusted the mixture, it revved smoothly and ticked over perfectly. I decided to do a test run up my driveway which is steep (one reason for the 72cc upgrade). Revved it up, off I went, nice and smooth, big grin, then the engine note changed and the power was gone, I pushed / helped it up the hill to level ground (grin gone) and it ran fine. Having experimented it seems that hills are the problem, the engine note suddenly changes and power is lost, sounds like it's coming from the carb! It still ticks over fine and runs up to full revs on the centre stand. Now I'm thinking about the crankcase to case seal being unreliable, no evidence outside but maybe below the reed block where it wouldn't be seen? Has anyone else any ideas?