Karting...
 

So, I stumbled onto this last night, and it looks interesting... People are using off road go-karts as a cheaper alternative to the $15-20k side by sides... They can be bought for under a grand, fully functional(used of course), and then, once the Briggs engine gets old, dirt bike/atv/street bike engine transplants are fairly straightforward and really will wake up such a buggy, once fully sorted... Most interesting swap I seen was a Honda VFR800 into an off road buggy... Not the typical CBR/GSXR/YZF-R swap...