I find that IA's comment interesting as I've been around these planes since their inception in the 80's but have never seen a Kitfox with an 0-200 for power. I admittedly have heard of a few, but kinda think if they worked as good as he described we wouldn't be putting up with the price or complexity of the 912 Rotax.
The Kitfox 4 really was not designed for the weight of a Continental. If that is a desire you need a Model 5 or newer.
But if you want a real hotrod Kitfox 4, just put a 100 hp 912 in it. Or go really crazy and do a 914 or a hopped up 912s turboed to 140 hp. You take a 650 lb Kitfox 4 with that much horsepower and that little weight and you will probably have an airplane that would either blow most people's mind, or maybe scare them