I have a small engine question.?

Hi, I have a small 3HP Briggs & Stratton engine and a messed up Sunl 125cc pit bike with a 4 speed semi-manual transmission, and I was wondering if the transmission would work on the engine or not, (even if I have to build a mount). Please let me know if this is possible, or if you have done it that would be great! I would love to build my son a go-kart for his 10th birthday. Thank You.