Lots of them use parts out of junked cars, but also "crate engines" are a thing, you can just buy a powertrain from any number of suppliers (for instance, GM: http://www.chevrolet.com/performance/crate-engines ).

And then there are specialist machine shops that build linkages and such to go between whatever components, at whatever length.

I imagine safety regulations and cost are the big blocker for small scale manufacturing.

If electric becomes a viable alternative to remans and crate engines the OEMs will just offer a "crate engine" electric drive-train that bolts where an LS or SBC would go and the stuff needed to make it plug and play.

You don't need a custom vehicle, just an S10 with a blown motor.