That would depend on which engine "look" you want.... The "baby knucklehead"
Or the "ashtray" head top....(the rounded-fully-finned one on that $600 bike)
Both designs came in 250 and 350 cc versions (different years), and the "Baby Knuck" 350 top can be used with the 69 through 73 ashtray 350 bottom end....(69 had the strongest engine cases, and the 74 Knuck top is virtually a bolt on in the 350 ccc engine)....They won't easily swap on the 250's because of differing stroke lengths.....74 and later 350's have an electric start as well......Lots of options depending on years....