I had the engine built in my car but that was it
And that only cause I couldn't afford it to go bang .
The rest I made or modified as my pockets ain't that deep lol
and fit it all up is part of the fun

To list a few I made the varible length trumpet/water cooled plenium
Inter cooler
Wheels
Adjustable top mounts
Half the cage
Alloy bells
Inlet scoop
Rear mounted rad install
Bias pedal box

And so on so on
so I would like to think I made mine

I feel you respect it more when it's made
Rather than just paying someone to do it

But that said some people should never be aloud to work
On cars lol