I've got a Karmann Ghia that I need to print on my FDM printer and from what I've printed so far know I will need to do some putty work.
The parts on the end were printed with the standard nozzle, but the middle piece was printed with a smaller 0.2 mm nozzle I bought later.
The .2 nozzle takes a bit longer but as you can see, the detail is much better.
Nicely done!
B-N-L Resins has a V6 that might work for you. They were MIA for a while but I've heard several folks say they've ordered lately and gotten quick turnaround.
Its kind of a generic casting but decent. It comes with a turbo but I'd ditch that.
http://www.b-n-lresins.com/shoppingopencart/Aftermarket-resin-cast-1-24-1-25-scale-model-engines-Ford-Diesel-Mopar-Oldsmobile-Toyota-Honda/Aftermarket-resin-cast-1-24-1-25-scale-hobby-model-Chevy-GMC-engine/Aftermarket-resin-cast-1-24-1-25-scale-model-model-Chevy-V6-Turbo-engine
The real fun comes in when you start designing your own stuff.
The side pods, block, heads, swing arm, and gas tank I designed and printed on my FDM printer.
I have seen online posts where they bought foil tape at dollar stores and got thinner tape than regular foil tape yet still a bit thicker than BMF and they liked it. Perhaps it depends on the source. The tape I have is way too thick for trim around windows etc. Also, the adhesive is extremely unforgiving. It will pull the paint right off.
Someone on FB said that B-N-L is back and they just received an order. You may want to check with them.
http://www.b-n-lresins.com/shoppingopencart/Aftermarket-resin-cast-1-24-1-25-scale-model-drive-train-brake-rotors-transfer-case-transmission-rear-ends/Aftermarket-resin-cast-1-24-1-25-scale-model-drive-train-rear-ends-custom-unique-Dodge-Ford-Dana-Chevy/Aftermarket-resin-cast-1-24-1-25-scale-model-model-Rockwell-Axles-Num1
I haven't seen any but those aren't the kinds of parts I've been looking for, as I do 1/24 and 1/25. I do think that designing your own in 1/12 shouldn't be that difficult. I did this PVVC conduit clamp just by measuring and basic cad drawing. I've also done a few simple model parts but just starting out.