In Poland the law allows you to do mods on the cars. You can see plenty of them on the roads, but there are some limitations like windscreen and other windows must be 70 and 75% transparent, and as far as I know it's not allowed to use xenon lights if there are not fitted by manufacturer. Also all parts you want to fit in your car need homologation. Obvious thing is that the modifications can't affect road safety, so use common sense and be think safe;)
Related: Starting a Auto Performance Shop In poland?
Performance? You mean performance?
Selling products from companies like Blitz, Greddy, Trust, Vielside etc?
I'm afraid that could be a non starter car modifiers here like huge exhausts and mismatched bodykits, Give me a shout if you have any luck.