More speed is probably going to mean more fuel burn, if you want a STOL design.
I you decided to move away from a Kitfox, you could consider a Bearhawk Patrol or Bob's new Bearhawk LSA. The LSA in particular is attracting a lot of attention. Here are some numbers, which by all accounts are the truth:
You're looking at a cruise speed of 140mph in the Patrol, with up to 165mph (Vne). With a Patrol you're hauling up to 1000lbs useful, and still the takeoff is only 250ft at MTOW and landing in about the same, landing speed is 35mph. Much less if you're lightly loaded. But that is with a more powerful engine, 200hp. With a more fuel efficient engine (115hp) you'd expect the speeds to come back a little bit, but so does your fuel bill. The useful load is bigger with a lighter engine too.
Now with the LSA, they are still working on the Quick Build kits, but I understand they're not far away.
You get 125mph cruise, Vne is 140, and the landing is even slower now at 30mph. They use a new aerofoil. Takeoff is 200ft with a light load and 400ft fully loaded. Gross can either be 1200lbs or 1500lbs depending on the category you're flying in, so useful load is between 450 and 750lbs. I dont know which engine mounts the factory will support, but I guess Rotax must be one of them. They are flying between 65-75hp engines right now, Bob was still working on it last we spoke. I bet you could put a 100ph Rotax in there if you wanted to really kick off the ground.
I guess it all depends on your mission, it might be a little more aeroplane that you're looking for at this stage? If you just want performance and waaay more speed, they'll do that.