Threebond 1194 is what you want. It's the replacement of the product that used to be sold as yamabond/suzukibond (has no lead in it now). Comes in a tube for about £22, apply it with a small brush so it goes on thin. Before you join the cases make sure there are no raised edges from prybar bodgery to hold the case halves apart, as air leaks spell death for 2-strokes.

The powervalve isn't essential to the running of the bike, in fact some bikes were sold with the valve present but no actuator - the valve was fixed in a pre-set location (the TZR125 was not sold with the actuator as a uk bike in fact). If your piston was damaged then that would be the cause of the very poor power at low revs, the valve just alters the port timing to provide increased drivability down low and high throughput at revs. If you had no valve, you would suffer from poor low down response but not to the level you are describing. Bottom line is - you don't need one for getting about.