WAYNE - Another idea I've used with some success is to buy a short pipe nipple in the size you need, 1-1/4" NPT, and saw a couple slots down the ends of the threads parallel to the center of the nipple, just like the flutes on the tap. It really won't CUT new threads but will chase some of the rust out of existing threads. Also I'd recommend NOT getting a closed nipple, too short, get something with an inch or so of unthreaded length.
Also, Kevin H is correct, roll the engine over BACKWARDS till the exh. valve is closed on it's seat, gotta get past the ACR on the exh. valve. Then there's no way chips, etc can get into the engine. No need to mess around with the spark plug & compressed air that way. THAT can be very dangerous, a few PSI of compressed air in a cylinder and the piston WILL go to Bottom dead center in a heart-beat and it won't care WHAT's in the way.
A tapered pipe tap is really what you want, not a Bottoming tap, for tapered pipe threads. The threads have to be a larger ID at the end of the port and smaller at the back of the port or the elbow will be very loose and never get tight. There is such a thing as "Straight Pipe Threads" but they need a "Special Dry Seal" male pipe thread form to seal properly. Granted your not trying to hold water pressure or anything but mixing thread forms is NEVER a good idea. Plus the taps will be more expensive.
Proper tapping procedure is to tighten the tap maybe a quarter turn and back the tap out a half turn, then tighten again an additional quarter turn. Don't force the tap to turn in too much because those BIG taps are really strong, they seldom break but I have seen it happen, but they also put a LOT of stress on the exh. port to expand the hole and can crack the exh. port.
Also may be best to NOT use the lock ring. As it tightens it will try to pull the exh. elbow out of the port. Once the muffler is installed the elbox shouldn't turn anyhow.
Also, not surprised at the price of those large taps. Once you get above 1" NPT they get Brutally expensive. You should check prices on NPT thread gauges on 1" NPT or larger, "Go & No-Go" gauges are $300-$500/set.
You might want to check with a local machine shop or plumbing shop to see if they have an old used almost worn-out tap they would part with.