The 125 twin was made in two designs........the 59-62 versions were known as the Benly Touring, Sport, and SS Racer. If you can find one........bring $$$. The later SS125A was produced from 67-69. It is a very sweet little motor that came with a CV carb. Its an affordable tiddler in the US.
Maybe you want to take a 350 or 400 / 4 and shorten the stroke at the crank, severely shorten the barrels, use modern 'ring-holder' pistons, and build a 250cc 4-cylinder that will scream 16,000+ rpms! Several builders have also played with the CB72 250cc twins by modifiying the cranks and putting early Suzuki DOHC heads on to make them scream. According to a retired HRC engineer, the secret to getting the 250 twin to pull well from corners was to balance the crank at 62% of the rods and pistons' weight. Several cam variants are available, some in the UK. Slipping a standard 6-speed tranny into the cases from a CL360 takes some machining but reportedly works.