ORIGINAL: spasmatic
The front edge of my firewall is 2 inches from the forward bottom area of the windshield. If you cut your front off, allow the sides of the fuselage to extend 1/2 to 3/4 inch past the firewall to allow for triangular stock reinforcement. Follow the previous angle of the firewall (slightly slanted) and it will also have to be cut from larger stock. Going back to the engine, mine is a smooth running as any glow engine of similar size.
Be sure to reinforce the wing attachment points and the empennage (tail group) with triangle stock.
With that much extra weight on the model, it will be easy to exceed the VNe (velocity never exceed) of the model even in a slight dive. The result could be folding the wing, failing wing attachment points or losing the tail group. Not pleasant to contemplate.
Good luck with your conversion and keep us informed of your progress. We need pictures too!
Ed Cregger