Just get yourself a 'decent' quality rear resonator/muffler replacement...
I say 'decent' because all the cheap stuff is nasty shite that will be too loud and not last very long and drone alot...
You've got to keep in mind that IL4 potters don't sound really good anyway compared to most other engine configurations, so you've got to watch out when increasing the exhaust volume on them, because with some types of muffler/resonator set ups they just end up sounding like a loud fart or simply awful...different story with a typical V6, V8, IL5 or even a boxer 4 potter...
On my turbo vert, I've got a Remus rear muffler specifically made for the TS vert with a Z20LET, the rest of the exhaust is stock and the difference is small, but noticeable, it is definitely a deeper note and only slightly louder on WOT (wide open throttle) the note is better than stock o/a and there is absolutely no droning anywhere in the rev or speed range...
Only down side is cost, all Remus products are not cheap, but in my case I wanted long life and quality as well as good looks and the Remus will give you those 3 things :)