Fast forward a few years from the 80’s to get away from the collectible nostalgia of the now middle aged’s youth.
Derbi were the sizzle in the early 2000’s. Senda was the ‘MX’ bike and GPR was the sports bike. Just as quick as the aprila competition, but a whole lot less £££ as there was no reflected glory from a 250 GP bike.
Any 50cc 2T will have been thrashed. At least something from 15 years ago has only had 13 years of neglect after its first unsympathetic owner, anything else mentioned has 30+ years of abuse and rust!
Or left of field if you’re mechanically minded, find an old bantam hardtail frame, and find a suitable engine to build a custom?