I would suggest you try and find a complete later model bike as it is getting more difficult to find missing parts these days and the cost of these parts has increased accordingly. Any of the bikes you have chosen will do the job even though they are all different to ride with each having their own personalities. I would personally choose a Bultaco because they have the best parts availability and they are relatively bullet proof and easy to work on. I quite like Montesas but they have some issues when you come to work on the engine, I have only ridden a couple of Ossas back in the day but they also felt fine and regarding the Yamaha, they sold heaps around the world and they were very competitive and reliable with good brakes. I would also recommend you think seriously before modifying any of these bikes as they were all developed by riders who were competing in the World Championship at the time and in particular the Spanish bikes were very close to what the factory riders rode the year before. Enjoy restoring the bike, make sure you put a good set of shocks on the back and a good set of tires as well as a set of late model wide footpegs and enjoy the experience

2 hours ago, sherpa325 said:

I would suggest you try and find a complete later model bike as it is getting more difficult to find missing parts these days and the cost of these parts has increased accordingly. Any of the bikes you have chosen will do the job even though they are all different to ride with each having their own personalities. I would personally choose a Bultaco because they have the best parts availability and they are relatively bullet proof and easy to work on. I quite like Montesas but they have some issues when you come to work on the engine, I have only ridden a couple of Ossas back in the day but they also felt fine and regarding the Yamaha, they sold heaps around the world and they were very competitive and reliable with good brakes. I would also recommend you think seriously before modifying any of these bikes as they were all developed by riders who were competing in the World Championship at the time and in particular the Spanish bikes were very close to what the factory riders rode the year before. Enjoy restoring the bike, make sure you put a good set of shocks on the back and a good set of tires as well as a set of late model wide footpegs and enjoy the experience

Maybe change the fork oil and check that the fork springs have not sagged too much.?