If I remember correctly, from a previous thread on this incredibly low powered race car, the turbo is there to maximize torque throughout the rpm range, and a flatter torque curve would make the cars less difficult to drive.
There must be some rule on maximum boost, because this could compensate for the small intake size. The proper way to lower compression would be shorter piston rod length, but there are probably rules about any engine mods, since this is a spec class.
I never heard of the MRT5 class of race cars before LFS, but I'm in the USA. In the USA, go-karts would probably be the most popular form of cheap racing. Then other than modified street cars, it would be Formula Vee's, Formula Fords, Formula Mazda (not to be confused with the new Champ car Formula star Mazda's which are similar to the Reynard, but use Ford-Mazda Duratech 2.3 liter engines modified by Cosworth with 300hp). Caterhams, Aerial Atoms, and Radicals are also popular, but these get expensive.