That sounds more up to my speed 😃
It's not so bad when you're working on older cars, but I can definetly understand the hesitancy of doing it on something brand new. Before year 2000 you could get away with using a tuner chip, but nowadays the ECU's are complex enough and integrated enough that you basically have to use a custom ECU, and as such many things stop working.
Inspections? Depends on your car and who is doing the inspecting. If you've got a new car with computers, they hook up the car, put a probe up the tailpipe and measure and read away. It's thorough. If you've got an old car like mine that doesn't even have a proper OBDII jack (It has the jack but was made to blink a diode for fault codes. Don't want to hook a computer up to that 😄 ) they do the typical shakeboard, visual inspection, prodding, physical braketest and smog. They check the odometer and record it each inspection, and they also check for dashboard warning lights and that your headlights and taillights work properly, and that the horn works. And they make sure that you have a warning triangle. If you were to roll in with a V8 swap under the hood, they'd require me to do a certification smog test. This is basically a write off for the car, since we're talking about the same test that manufacturers have to do for their new cars. That's a few million bucks. If you roll in with a tuned original engine and the inspector is a gearhead (They typically are) they don't really care aslong as you pass the smog, which is easier on an older car.
TL/DWR version, pre 1987 cars are easy to mod, not so much after that.
As for what is allowed to be modified, that depends on the age of the car actually. We've had a couple of different car registration "eras" so to speak. Before, iirc, 1971, the only thing that was recorded was power, weight, length, width of the vehicle. So my 1968 Volvo Amazon I could swap a V8 into and they can't say anything about it aslong as it doesn't produce wildly different amount of horsepower compared to the Volvo Amazon maximum, the only thing I'd have to do was register an affix for the papers that said I'd done an engine swap. After that they added some more things to the papers, I can't recall all what it was though but I think tyre dimensions was added, and in 1987 catalytic converters were mandated on all cars, so if you want to do an engine swap on a newer model than that you have to include a catalytic converter. In 1993 they tightened up the registry with more details again, environment classes, gearbox type, noise level, rims and such, and this is the last year that you can register an engine swap on a car. That's why I got myself a 1991 Volvo 240. After '99 or something like that they recorded almost every single little detail about the car, and you're not allowed, technically, to deviate anything from it.
There is one way around it though, you can register the car as a total conversion. Then you can do whatever you want basically, but you have to pass a special registration inspection where they absolutely make sure that you have done a total conversion and not just an engine swap or similar. You have to modify the engine, the brakes, the drivetrain, the suspension and so on in a meaningful way. The only thing you don't have to change is the bodywork.
And I've only touched lightly on the subject of modifying here 😅
My fathers Volvo 740 passed the inspection with his 400+hp race engine in it without issues, the inspector just looked at the turbo and chuckled "That's a big ****in' 'un, heh".