Judging by how they machined the outside of the cylinderhead, they probably wanted to fit valve-stem-seals from some sort of car. If you still have the old ones, I'd go to a car shop and show them to a mechanic and hope for the best.

Otherwise: As Ali said, you'd have to get some new valve guides. Stock Yamaha stuff is expensive, but if you look around a bit on ebay or buy them directly via one of the Yamaha parts wholesalers in the states they maybe can be had for less.

And you may want to search the net with the original spare parts number for the 1st o/s valve guide:
4X7-11133-10-00
Which will hopefully help you in finding some better results.

Cheers,
Greg

Valve and combustion chamber sizes (750 vs 920/TR1) differ quite a bit. 750 valves and combustion chamber are a lot smaller. I don't think the casting marks mean anything really.

Cheers,
Greg