any 1960's IH experts out there

I am ivor from UK ..I live in mid wales and for a few months I have been helping an elderly farmer a few miles away .. he has a 1960's forklift truck I am not sure of manufacture but it is based on a 434 or 276 IH tractor. the problem is with gearbox it will go in reverse and neutral ok but when it is put in forward gear the engine is stalling as if it is in two gears I have removed top of gearbox and I notice that one of the gears has lost parts of teeth all around one side also the high/low lever is stuck in low but should this cause problems. thanks in advance

I would say you have found it out yourself probably one or some of the gear teath are stuck in between gears.

Could be several problems...

The gear with the bad teeth on one side, that sounds like a gear that gets shifted in and out often, and has been roughly forced into gear a few million times. As long as no teeth are completely gone, it may not work well, but should still work.

The most common reason for a transmission to lock up is being in 2 gears at one time. That can be caused by the shifter getting out of it's proper shift gate position, or a broken shift fork, or broken shift shaft. Try jacking one drive wheel off the ground to relieve any load on the gears, block the clutch down, and reach in the top of the gear box with a pry bar, see if you can get it into neutral. Look at the shift gates, they should be lined up where the shift ball moves them back and forth. If it's in neutral, and the gates are not lined up, something is broken or out of position, a loose set screw where the fork attaches to a shift shaft, etc.

Then it can also be a broken gear, a seized bearing, a foreign object fallen in jamming a gear. With it jacked up, some careful examining while rocking the wheel back and forth or turning the input gear, you should be able to spot the problem.