ansys, the vacuum check valve relocate is no big deal. It is just a 10 mm one way valve fitted in the hose coming from the engine intake manifold, and I used a Honda one. The hose in the front trunk just connects directly to the booster.
 
On my '90 model ADM SW20 NA I still have the early small brakes that I have found totally adequate, however in an attempt to get rid of the 'springy' brake pedal feel, I fitted a '97 turbo booster with a one inch diameter brake M/C.
The result was just what I wanted with a firm pedal and much shorter travel, and not much pedal pressure needed. 
 
BTW, I have run the car with both the '90 booster and the '97 booster and found that even though they look much the same the difference in power between the two was quite dramatic so the internals are obviously different!!
 
Also be aware that I believe SW20 boosters were changed to more powerful ones after 11/91, and then to even more powerful ones after 11/93, but I don't know what the part numbers are.                                                                  Unfortunately there are no markings on the '97 booster I fitted.

I've also done the change from early model non ABS braked car to later model discs, calipers, master cylinder, booster and proportioning valve. Finally found a 15/16 inch master cylinder to be optimal with a large diameter (slim) late model booster. I can also recommend Forza FP3 pads from Brakes Direct. I placed an all steel vacuum check valve close to the inlet manifold. I used genuine thick walled vacuum hose too !!