Briggs & Stratton

Guys,
I'm building a mini-bike for my son, and I have a 5HP Briggs to use, but it has a 1" diameter output shaft on it. The clutches they sell for mini-bikes are made for either a 5/8" or 3/4" diameter crankshaft.

Does anyone have a Briggs 5 horse crankshaft that is either 5/8" or 3/4" diameter that I could buy inexpensively? If so, I could simply swap the crankshaft out to the correct size.

Thanks in advance for any help.

George