I would use a drill bit and drill all 4 corners where the recessed box will fit. I'd use the small pointed Zona saw blade to make the cuts, and then build the new engine box to fit the opening. I'd then tack the box in place and cross drill the firewall and box to accept wooden dowells. I'd then set up the box to accept the engine mount, and once all the blind nuts, holes for fuel lines and pushrods were drilled, I would epoxy the box into position, and epoxy the dowells in place. Once the epoxy has cured, trim off the excess dowell, and sand smooth. Finish with the appropriate colored butyrate dope to fuel proof the new firewall, and you should be good to go. engine setup and balancing the model should be straightforward from that point.

Adding weight to the nose is standard fare with most all WWI birds and others with a short nose moment. Big ol' battery packs are better than a lead lump.

Bill, Waco Brother #1