It really depends on if you've done anything to your "el-cheapo" so far.
If you haven't done anything... you can get a better carb, expansion chamber exhaust, replace your intake and exhaust manifolds and re-bore your reciprocal jug ports to match,
replace your jug head with a high compression milled or cast cylinder head (crmachine or puch are good options), get your crankshaft balanced, replace your billet rod, etc, etc
(Yes, there is actually more, much more)
Now, I haven't owned any of these, BUT, if you really want to squeeze out everything you can from these, expect to pay for it unless you can fabricate / modify some of the parts yourself...
That being said, there are TONS of parts out there and documentation on the aforementioned.
Alternatively, you could purchase a different motor....
Maybe the best question is, what specifically are you trying to accomplish, and how much are you willing to spend to achieve that?