There are the 2 big screws at either end and then there are a number of long captive bolts attached to the spoiler which poke through into the cabin and the nuts are hidden under the trim above the rear screen (you'll need a deep 8mm socket).
My first Mk1 did not have the captive bolts and was just held on with the 2 big screws on either end, never fell off.
From what I recall the captive ones are M5 and are attached to tabs on the fwd edge of the spoiler, you won't be able to see them without removing the spoiler.
The screws on the side are M6 but they have a big fat head on them which will probably be a Toyota only part or a few pennies from a breaker. There's a chance I might have a couple in my box of random bolts, I'll have a look this eve.