Hope the advice helps out. Just don't weaken the stress areas where the booms and center section go together. You need it to be ridged.
As far as the budgetary constraints go I can completely understand! I usually compare horsepower, prop size and RPM. I would also send out feelers in RC universe for opinions comparing your engine choices. That way you can make a informed decision. I would think a .72 may have more power than a .91 4 stroke as the .91 is generally equivalent to a .60 2 stroke
Good luck[8D]