I ran across this site yesterday and found some interesting 4 stroke engines for sale.
Small Engine Warehouse - Repower Outdoor Power Equipment
The motor I am interested in is:
The Robin America SP170-SD
6HP Horizontal 3/4" x 2 5/16" Shaft, Fuel Tank, Muffler, Return Spring Throttle for Go Kart
Product Number: EX170DT0011
Overhead Valve Design
Electric Start: No
Fuel Tank: Yes
Oil Capacity: .60 liters
Oil Type: 10W-30
Shaft Size: 3/4"Dx2-5/16"L
Made For: Go Cart
Depth (Less Shaft): 9.5"
Shipping Weight: 40 Pounds
Warranty: 3-Year Manufacturer's Warranty
Returns: Limited Allowed
Our Low Price: $235.00
55 In Stock (Subject to Prior Sale)
They have a lot of other choices and you can select by horsepower range. They also carry clutches and parts for go-karts.