I would think that helicoil inserts could be used in this application.

If the area/space near the exhaust port is open and would permit you to thread a "universal" type of muffler with 3/4" threads, then, yes......you can use one of those screw in mufflers.
Be sure you clean those threads out in the head before you attempt to screw the muffler in. Preferably by running a 3/4" pipe tap into the hole.
I know it's a tall order for somebody to have ready access to a 3/4" pipe tap, so I will offer an alternative tool.
Make yourself a "thread chaser tap" using a 3/4" pipe plug made of steel, it has to be steel to be effective.
With a hacksaw, cut some slots PERPENDICULAR to the threads, as deep as or deeper than the threads themselves.
Usually 4 evenly spaced slots are enough.
The slots provide a "scraping edge" that will cut the accumulated carbon and other deposits loose from the head and assure a good fit for your new muffler.
You will do a better job if you buy a pipe plug that has a hex head, and use a hex socket to turn it with.
Start the tool by hand and then turn the tool in about 1 complete turn, then remove it and clean out the threads and slots.
Repeat the process and advance the tool about another half turn each time you thread it in.
If you don't clean out the threads before trying to use them, you run the risk of having the threads strip out of the head when you start using the engine.
And be sure you use a lock nut on the muffler threads (it might come with one, if not.....buy one).
See image below for mock up of thread chaser tool.

This post was edited by mownie on Wed, Aug 14, 13 at 11:54

thanks for the easy alternative ,my npt tap is not 3/4 but plugs i do have.