Scott Dinger makes a tube style muffler... the ones that Tom Lay recommends for his .40 Red Heads.
They are not exactly cheap but probably one of the best money can buy, engineered to not overheat a McCoy.
Scott's moniker on SSW and eBay is "sq8s"
As far as NVA goes... that is about as easy as finding hen's teeth. You might consider an after market Super Tigre style, or if your a bit eBay savy your can find a low buck, burned out (and sometimes not burned out) engine for parts, including the NVA! Been there, done that... Last week I was the winning bidder on four engines that came to $17.80 total including shipping. Two Cox 049 Baby Bee's, one .020 production engine, and one near mint (although a bit dirty and gummed up) McCoy .35 Lightning Bolt. I crock potted the McCoy and it cleaned right up, a fine engine indeed! I will sell the Cox engines to get my money back and should just about end up with a free McCoy .35 LB (I love it when a plan comes togther)