I enjoy the old V12 too, but there's definitely not alot of aftermarket support for it. Boring it that amount will not increase it a full liter, either. Realistically, the only way I know of getting custom pistons is JE or Wiseco. One of those two will make them if you send a stock piston.
As far as turbo setup, well that's up to you. I've never encountered a kit for them, but any fabrication shop can make the manifolds and down pipes. That's the serious fabrication side of turbocharging. Intercooling is simple, buy a universal intercooler, and 2 sets of universal piping kits, that usually does it...
Turbos that are Garrett flanged, most common are T3. A couple GT28rs's would be great, and a responsive low boost engine. With forged low comp pistons, and a good tune on Haltech, Megasquirt etc it would be nice.
It's all custom on odd-ball apps. The 850 transmission works, yes.. But, would take work to make it go in a E36 I would imagine.
It's been done in E30s, but it's a tight squeeze. Honestly, it is a heavy engine with a major lack of support. But if you were to do it, it would hit like a sledgehammer!