I've had a similar interest in putting mufflers on my existing Rossi, Conquest, and Nelson engines so as to utilitze them in F2d.

I'm afraid this is an uphill battle to say the least.  The current $300 F2d engines were designed with mufflers from the ground up and they weight half the weight of a retrofitted old school engine.

I'm still thinking of giving it a go for the sport of it, but I don't expect to be competative. 

For only $1,200, you can get a four-pack of the real deal, so don't sink too much into the old motors.

Plan B might be to find (or found) others events for the old 15's.

Steve, you'd best check the dB's.  F2D "mufflers" are pretty token when it comes to making the engines quieter.  Most of the noise actually seems to come from the prop and intake.

The F2D rules are pretty specific on the size.  I did some measurements, and a stock MACs header pipe is just about the right size.  All it needs is a proper size stinger on the outlet.  The dimensions are >12.5 cc's, and less than 15 cm from the piston face to the end of the outlet, which has to be smaller than 8 mm. id.

What I'd do instead is use a mousse can muffler. Fairly easy to make, no need for welding, but of course it won't be as nice a fit on a RTF F2D plane as a F2D muffler would be.

This could just as easily be posted on Carrier, Scale, Stunt, or Combat, but you asked it here, so here goes.....

This setup has survived several bench runs and three flights. 

It:

Reduces the noise level.
Make the engine idle better.
Satisfies F2d rules with it's 8 mm oraface.

Materials: 1" square tubing from Home Depot, Permatex hi temp gasket sealer from Pep Boys, silicone sheet from McMaster (not really needed), 1.5" 4/40 bolts.

ps: This not my entry in the muffler and test stand beauty contest.