Hey, engineering certi would be helpful in regards to the EPA well it depends on how loud you car is I believe there is different db for each car as long as ur under that u should be fine, the certi would cost id think around 700 roughly depending on were u go I was quoted that for a 13bt in a holden gemini conversion and 300 per each re test as long as its satisfactory safe, as in rested for torque of the car and power has and xxx amount of body movement n so forth, I believe it would be maybe worth it if its ur pride n joy but it depends on if its going to fail tests n so forth considered putting some of the stock parts back into it getting test n re putting em back in? much cheaper way if u have the stock parts still. around 8yrs ago awhile ago my mate had cops go over hes car and had alot of dodge **** to hes car n should of been off the rd but still let him off after he produced a engineering certi whoobley seat blown turbo bald tyres maybe just lucky but as soon as the certi was produced it changed them from being harassing asses to not so.