Hi hard y rd,
that one is in sad cosmetic shape but even so the asking price will be not much less than $2,000.
It will take a young fortune to replace the missing sheet metal parts and get it back to stock condition.
OTOH, as long as it runs you can ride it "as is" just fine as it don't cross PEI's anti-hardtail legislation.
Google "CVMG" to find the Canadian Vintage Motorcycle Group website and scroll down to the "parts for sale" column to find all manner of new and used BSA parts.
You could also seek out the BSA Owners club.