I also own a Bultaco. A Model 98 ('73) Alpina 175. It was, quite literally, a rolling basket case when I bought it off eBay four years ago. I stripped it down to the frame and completely rebuilt the engine and suspension. It had a 250 top-end as did most of the 175 Alpina's sold in the U.S. Glad I found that out before ordering the piston and rings.
Rode it at a few Spanish Motorcycle Owners Group (SMOG) East Trail rides, but recently converted it into a "Sherpina" for vintage trials. Fitted it with a model 91 Sherpa T "Slimline" tank and seat, plastic trials fenders, trials bars and lower gearing. Still have the Alpina tank and seat, stainless steel fenders and steel bars. If I ever find some stock lights and speedo (cheap) I may turn it back into an Alpina and build another vintage trialer.
Also thinking about building a Pursang for vintage MX, but after a few "Oh sheet" moments at the SMOG rides (and a broken finger), I'd build a left-side shift/right-side brake Pursang.