The stock 914/6 brake package is sufficient for street/mild track use.....when up to snuff. You may have the infamous 17mm master cylinder in there which will cause "sponginess". New, modern pads, a new 19mm MC, rebuild the calipers, round file the proportional valve (an adjustable one in it's place), flush the system and you're good to go.
Any brake system sitting for 20 or so years is not trustworthy.
BTW, those stock 914/6 rear calipers are worth gold.....don't get talked out of them.
BTWII, you'll find 914s, mild to wild at
www.914world.com