You can spend a little, or a lot. It all depends on what power output you are looking for. Up to 100 hp, I would stick with the wedge engine you have. Header, sidedraft, bit of porting work and you're there. A lot easier and cheaper than fitting a similar output crossflow. You can get even higher outputs from wedge engines but then reliability and street drivability suffer.
The nice thing about a crossflow is that you can build a 120 to 140 hp engine that is as sweet as a nut to drive. Tractable and economical (at part throttle) and plenty of top end -- forget about catching 300hp soccer moms though. The catch? $$$ It isn't cheap buying pistons, rods, valves and tons of machine work. The higher the performance of the engine you start with, the better. Already has 10.25:1 pistons? Great. 9.0:1? Add pistons and liners. Large ports and valves? Excellent. Just need a bit port matching and smoothing. Small valve and port head? Lots of $$ in parts and machining. It can all get expensive in a hurry.
