Allis Chalmers 808 GTII lawn tractor-transaxle question

[totallymopar]

I have an Allis Chalmers lawn tractor,1976-82 vintage, Model 808 GTII.It has an 8 HP Briggs & Stratton engine with 3-1/4 drive pulley,and the 3-speed gear transaxle has a 9" pulley on the side. How can I identify what tranny it has,(mfg.)and where on the unit the I.D. is located. Also,I want to up the speed to around 20-25 mph,to use it as an ATV. I'm thinking a 4-3/4 on the engine,and a 5" on the tranny. With a 3600 max engine RPM,the axle pulley will go from 1300 to 3400 max RPM----2-1/2 times faster than stock. Can I do it with this tractor,without grenading the tranny? Any other info on this tractor would be appreciated.