Back when I started racing my car in the class that morphed into Improved Production, the rules said the car needed to be road registered. This was in the late 80's though, so getting the car registered was simpler. The class was pretty simple, with the same engine rules as IP, full interiors, centre consoles, & LSD's allowed but no welded diffs. So I been there before. Did it all again breifly in the late 90's when I raced in hillclimbs in my Sunny & it was road registered. Wasn't til I bought a cheap Navara & arranged to borrow a friends trailer that I deregistered the car. The main thing I always did was make sure the radiator was as big as possible. Rough riding, noisy, difficult to drive in traffic, highly visible to the police, all these I could live with. Overheating, pumping out all the water & making more steam than a steam engine, not amusing. So just make it overkill on the engine cooling, & the rest is just character. You could also check into mufflers, the Sunny had a hi flow muffler that was very good at low speeds, nice & quiet, but had a nice note at speed, without too much restriction. Putting some carpet back in will quieten it down abit, too. You'll soon be cutting them up on the on ramps like you do now on the track.