The first thing to know is exactly what makes an SPG different from any other 900.  I recommend taking a look at Larry West's excellent Guide to SPGs to help familiarize yourself with the core differences.  Assuming you can pull together the requisite parts, creating your own isn't too much of a problem.

To get that SPG look, you'll need:

•  The wheels - All SPGs had three-spoke, 15x5.5" wheels.  Make sure you get the right bolt pattern for your car, as the SPG wheels were available in both early and late style patterns.
• The side moldings - All SPGs featured "aerodynamic" side moldings unique to the model.  There are a total of ten individual panels associated with the kit - each side of the car features a front bumper extension, a front quarter panel, a door panel, a rear quarter panel, and a rear bumper extension.  On 1986 and earlier cars, the front bumper extensions incorporate a unique front airdam; 1987 and later cars have extensions very similar (but not identical) to the stock Turbo extensions, and use the stock Turbo front airdam.
• Fender Flares - Starting in 1988, SPGs received 1" wide fender flares which attach over the stock black fender trim.  Earlier SPGs did not have these wider flares.  Either way, attaching them is a matter of attaching some trim clips and the fender flares.  Do not that these flares will likely interfere with tires under full suspension compression if you're using anything other than the stock wheels - you may choose to leave them off.
• The side molding mounting hardware - The SPG panels are attached to the bodywork with rails, and the rails are isolated from the body with rubber insulation to prevent rubbing (which could cause rust).  Additionally, there are 1/4" thick rubber insulators which attach the panels to the inside of the wheel wheels to guard against mud and water.  Normal 900s have mud flaps and deflectors here, and they are different from the SPG insulators.  Make sure when purchasing or otherwise acquiring the panels that you get all the mounting hardware and the rubber pieces as this stuff  can be as hard to find as the panels themselves!

As long as you're starting with a car built in 1984 or later, you will only need to do extremely minor drilling.  Attaching the airdams, bumper extensions, and most of the panels require nothing special - all the holes and mounting points exist in the body work.  The only exception to this are five rivets which attach the door panel to the door's lowest edge.  You will have to drill positions for these rivets or the door panel will not fit securely.  Test-fit the panel, then use it to mark the correct spots to drill.  Some have suggested using a weatherproof epoxy to attach the lower edge - I've not tried it, but it should work in theory.