You're in luck in that I had to do all the research myself. The original was riveted to the brackets, but I developed an alternative method that just uses nuts & bolts. Here's a copy of my original post:

I got my NOS topper in the mail, and my size-matching for screws worked perfectly with the brackets for attaching the topper to them. First the brackets again:

http://www.marcospecialties.com/pinball-parts/535-6072-00

The screws that work for attaching the topper to them:

http://www.marcospecialties.com/pinball-parts/4004-01003-06

And the locknuts:

http://www.marcospecialties.com/pinball-parts/4404-01119-00

Now all I need to do is source a correctly fitting sheet metal screw for the base of the bracket that secures the topper to the backbox. I'm guessing something simple like a #6 hex since the hole is larger in the base of the bracket, but I'll find out to make sure and report back.

(followup post below):

Got the assortment of screws in from Marco. The #6 Hex Sheet Metal Screw at 1/2 inch long ended up being the perfect fit for securing the brackets to the top of the backbox. The antlers are now installed on my machine after having been missing for who knows how many years. They look great!

So here is the link for the hex screws:

http://www.marcospecialties.com/pinball-parts/4106-01088-08