Putting in a 4 stroke at the high end of the engine range is almost always a bit of a problem with many ARF's as they are usually designed around 2 strokes. Yes, it takes some modification but if you anticipate it and accept the fact it usually works out. I put together a Goldberg Tiger II for a friend and had the same problems with the throttle linkage which was barely solved with a Dubro linkage. You will find that if you look at an H-9 design with a radial engine type cowl things go a lot easier (Hellcat or Corsair). But in truth you can run into the same problems with kits and if you don't think it through early on you wind up in the same boat. It's like adding floats to an existing aircraft....you know you will need additional nose weight but you're never sure how much until you're finished.

PS- the new OS alpha 4-stokes are generally smaller and easier to fit into tight spaces than some of the older 4 strokes.