The little mini in the photo is a 3hp with a 5/8" crank...I also have a 1971 2hp with a 5/8" crank that looks just like this one except it's bright orange...they are both fun to ride, and work fine for my 240lb rider weight.
The frames are a little small for my 6'4" height though...
The 3hp works well with 12/60 gearing...the 2hp likes 12/72...
If it's a Briggs & Stratton 2hp, 2-1/2hp, or 3hp engine, then they're not rare...they were made in the tens of thousands, and many survive today and can be found at swaps and fleas...I paid $20 for the orange one, without the edger it was mounted on. The black one was $50...it was barely used. New retail for a 3hp is $199...
I've bought several of these little engines, and used to also buy the Azusa minibike kit...we'd put them together in a weekend, and they were tons of fun...the neighbor kids always talked me out of them quickly.