It's been a few years, but I've spent many hours with these cars and can definitely give you some advice.
Before anything else, you need to replace timing belt and idler pulley bearing. These engines are a heavy interference design and a snapped timing belt will cause bent valves (best case scenario, worst case is a total write off of the motor). The idler bearing is notorious and seizes up causing the belt failure. Looks like both are available on Rock Auto.
Window regulators are NLA and also prone to failure, I've got no advice on that just a heads up.
As for "hopping up", I would say it depends on what you want to do with the car and your fabrication skill level. Suspension in the GTA is about as stiff as you would ever want for a street car. I don't have any experience with the convertible, it would probably do well with some additional chassis bracing.
The rear axle has some camber built into
it and stiffer torsion bars and bigger sway bar. The front had stiffer springs and bigger sway bar. Magazine tests of the era showed the GTA skid pad numbers on par with Corvette. With some good rubber and everything in decent condition, I think you should be fine as-is.
That being said, if you wanted to go "all out" For suspension, My last Encore that I raced in ITC used:
-Bilstein struts for Mk2 VW Golf, with modified knuckle attachment flange holes and spring carrier removed.
-ground control upper camber plates
-coil over sleeves with 600# 2.5in racing springs
-no front sway bar
-stock rear GTA axle, torsion bars and sway bar
-Bilstein shocks for Mk1 VW Rabbit with spring perch removed and custom fabbed upper bushing mounts
The GTA has a pretty close-ratio trans and shorter final drive that helps take advantage of the power available. The 2.0 motor is pretty pathetic by modern standards but held it's own in 1987.
The 1.7 and 2.0 engines are almost identical, but the 2.0 is both bored and stroked to get the displacement. Heads and valves are the same, but the GTA had a hotter cam. Intake manifold and FI unit are also GTA specific, with a larger throttle body and higher flow injector. A euro-spec intake manifold with a simple Weber carb is probably an easy way to make a little more power.
There are no off the shelf" headers available. The Spec Racers had one, but it doesn't fit in the car. Getting rid of the cat and freeing up the rest of the exhaust definitely helps, though. Getting rid of the power steering also frees up some more power.
Hope that helps, that's a pretty awesome car you've got there.
-Jim