I always keep a supply of the Brodak mounts for the .049 engines on hand for spurious projects. Like Larry says, already has molded lines for the landing gear. Has molded tabs for built-in right thrust. Can be used for either tanked or horseshoe backed Cox engines (or engines sharing same bolt pattern). I like them, and they'er cheap. Saves a lot of building time.
That being said, you could just take the Brodak plans and cut the nose back to the desired length, then cut 1/4" balsa cheek blocks and ply firewall as shown on the Outerzone plans. I've not looked at the Stuntman 23 plans, but other Brodak 1/2A plans show where to cut for radial mounted engines.
Mark