Unfortunately, the cheapest easiest way to get 20-30hp is an engine swap. Pull the M117 that's in the US car out and drop a Euro 450SLC engine right in. It's good for 225hp which is probably 40hp over what you have now.

They are higher compression motors and the difference is in the pistons so you're not going to get there with your engine unless you pull it out and take it apart.

__________________
With best regards

Al