This can be done, but it's not trivial. Not super-complicated, but not a walk in the park, either.
But first, you need to make sure that there is no pitting on the block, usually at cyl. 1 (closest to firewall), at the coolant passage(s). If the block has pitting, then a new HG won't solve your problem. (This pitting problem pertains to B212 blocks only, NOT the B202s.) Saab's official fix is JB Weld but a longtime tech has posted on a forum that it doesn't work when it's right on the coolant passage.
More info on the block issue here:
http://townsendimports.com/Web/2point1_Block_Repair/saab_2point1_block_repair.htm. Personally, if the block is E36 M3 and I didn't want to chuck the car, I'd put in a B202 replacement engine. If you wanted to go Turbo, you can do this with your car without too much trouble.
Meanwhile, let's assume the block is fine. Your NG900 head will bolt on but it is shorter than the B212 head, so there will be a gap at the firewall end of the engine, where the timing chain passes down to the crank:
EDIT: can't seem to link to pic, so see here: saabhippo blog
You can cover this hole either by welding some aluminum on the head, or, as I did (well, a machine shop did it), mill out and epoxy in the aluminum. This is not structural stuff here: you're just keeping the oil in the engine. It's been a few years, but I think I paid $80-$100 for the work. You can also modify the timing chain cover, but IMHO it's simpler to do the head.
Pic for you:
There are two pieces of AL there; they may not look it, but they are flush.
You then need to make brackets for the PS and AC stuff because they used to bolt to the old, longer head. Here's a pic of mine; I have no AC so the bracket is for PS only. It looks a bit flimsy compared to others' designs but it's not doing much work and has held up well:
EDIT: can't seem to link to pic, so see here: saabhippo blog
You also have to deal with the intake ports, which are different than you B212 manifold: the head has to be port-matched so its square ports match the oval intake runners. My car's manifold was not re-drilled, but usually people fill the holes, re-orient the manifold WRT the ports, then re-drill. You cannot use your NG manifold as it does not account for the 45-degree layover of the block.
Use the original B212 cams and gears, a B212 gasket, and the original temp sensor. Exhaust manifold bolts on. Either modify the RH upper engine mount bracket to use both upper bolts, or just use one and leave the bracket as-is. This is the bracket with two 10mm bolts with 1/2-inch spacers, at the top of the mount, where it mates to the head. Your C/R will go up a bit. Chamber volume on the NG head is a lower by a few cc's (4, IIRC).
You can find more info on this swap at www.saabrally.com.
