Hi,
I discovered many years ago that the fork top bolts used on early Suzuki 50s and 80s (the Mark 1 M12/M15 and K10/K11) are identical to the ones on the BS50, BS60 and BS90. I can't find the appropriate Suzuki part number, but I'm sure there must be an early Suzuki man out there who could help.
The later Mark 2 Suzuki 50s and 80s (the ones withthe teardrop tanks), the AS50, and the T125 use a similar bolt with the same length and thread size but with a larger sized head - these may be useable on the BS50 Sport and BS90 if you're not going for originality.
Cheers,
Graham