I don't have any personal experience modifying the stock mufflers but know it was common practice for some people and they had good results. They were still quite heavy though which is probably not a problem on your warbird. You might be able to find some old threads that describe the modifications. Regarding aftermarket mufflers, I have flown with B&B, Slimline and Bisson mufflers on the Zenoah twin and all have worked well. The B&B is not much more than an exhaust diverter though and is quite loud but they are reasonably priced. Today, I'd look at the Bisson or Jtec mufflers as both are of good quality. I think you'll find most of the aftermarket mufflers are much more compact than the stock ones. Both of these companies custom build mufflers so maybe you could get something with an exhaust location more suited to your airplane.