Why don't you test first some of the easiest suggestions first?

fly the plane without cowl....low nitro fuel> that it does work ok in the other engines doesn't mean that it is good for all the engines, many engines are designed a little bit different even though they share the same parts and principles...the have different ports, some are ringed, some are abc, some have higher compression than the others, they behave different to nitro content or nothing at all.

Many of you are taking too much attention to the cowl stuff, and your cowling is not bad at all, but IMHO the problem comes from inside. Overheating and lean runs are related most of the time, and I've seen that the engine stops first due to the high temperatures rather than blow the plug first.... if the plug burns out it is due to wrong heat range, wrong fuel, and less likely from the quality of the plug, most plugs are virtually the same, differing in heat range, the size of the coil (longer on 4 strokes for example), but most of them use the same materials....

Lean runs and higher nitro than needed may cause extremely high temperatures.

Super tigre, like most of the euro eingnes, run well on low to no nitro. You can try buying a methanol quart and some castor from drugstore, it won't cost you much, and would be an easy test...

Check that the muffler nipple is tight.

I hope you get it to work properly...

best regards/saludos

Jorge