I'd use the 1.20 4-stroke. I built 3 Goldberg Ultimates in the last 10 years - all were powered with Enya R1.20GP 4-stroke engines with a Don Harris pitts smoke muffler. No firewall mods. I used the stock 2 piece mounts. Engine mounted as far back as possible - carb is 1/16 or so from the firewall. I have to remove the valve cover when installing/removing cowling. I mounted a 1200 mah battery in the tail (saved the cutouts for lower aft fuse and made a door). Good luck. You'll love the Ultimate!