If it's all original, then it has a 10hp Tecumseh.
The Comet clutch is a 780 and the shaft is 1" so upgrading the engine is pretty easy.
It is either model 612-00 or 612-01...here is a link to download the manual with repair parts:
http://www.bmikarts.com/PDF/612_operatorsmanual_6120061201.pdf...