Shame the Saab lump won't fit - 140bhp of turbo-nutter engine sounds good to me!!
Re the 1990 900 models, some of the turbos were 16 valve, if memory serves they were badged "900 Turbo 16" in the UK, not sure about anywhere else.
The "original" Saab turbo was an 8 valve, breathing through a single 175CD Stromberg and i think the basic engine might be possible to squeeze under the bonnet of a 7. That said, i'm not sure there would be the room for the turbo and all it's plumbing, not to mention the fact that as has rightly been pointed out, Saab put the engine in backwards and use a transaxle with combined engine/gearbox sump.
All in all, for 140 bhp, not really worth the grief! Would be easier to get a Montego (or Maestro but they're rare!) Turbo engine and fit it for 150bhp. I don't know if the Maestro/Montego ranges made it to the other side of the pond, but they are a family hatchback and saloon range with engines from 1275cc to 1994cc, the latter being the "O" series which was also used in the SD1 so will easily mate to the LT77 5 speed TR7 gearbox.
Maybe worth a thought? Or if you wanted serious fuel economy and weren't too worried about the traffic light grand prix, an "O" Series turbo diesel, still built under the name Perkins Prima so spares are plentiful!
Cheers,
Dave