I used a Fox 35 in mine initially.
However, it started misbehaving and I replaced it with a Brodak 40.
Not knowing any better, given that there was no guidance on where the engine should be placed in the instructions that came with the model, I used the Pat Johnston drawing to put the prop where the drawing said. That left space between the Fox and the firewall.
Now, the Brodak 40 sits against the firewall and the mounting holes straddle the rear mounting holes for the Fox. The balance remains the same.