hello mr Johnson,
Back in town? How was your Spanish holiday?

For a scale project, all that power is of secondary concern. Main issue is how to cram the largest possible engine in those confined spaces. More cc is more torque to pull large scale-like props. Climb angle of the Spit needs to be 45° at best.
Still, the engine needs to shift back as far as managable, and a prop extension must be added, so the carb will clear the cowl lines. A Mk V Spit in desert trim would be the way to go in my book.
For a muffler, a steel pipe section with front and back covers (alu) will do, if made into a motor mount. Weight up front is needed anyway. If it has to be made, it is likely to be a bit more expensive than the 3271 side muffler, but the results are so much more rewarding.