This link doesn't have your Gilson tiller, but maybe one of the manuals available will be somewhat helpful?

http://www.rustytiller.com/manual.html

I have a model GIL 1589A I just picked up. It does not run, what type of spark plug should the 5 hp B&S motor use?
Thanks

Champion J19LM or RJ19LM

Here is a link that might be useful: list 2 to 5 hp parts

I have a Montgomery ward front tine rototiller model # gil1580g that I am restoring for my father. I need a parts manual to start. Any and all help I could get would be greatly appreciated.

From an old Gates manual, the belt numbers are:
Forward 6835
Reverse 6737

Kind of hard to help you if you don't enable the email feature on Garden Web. Bill is right about the belts. The v-belt trade sizes are 3L370 for the reverse belt and 4L350 for the forward belt. The Gilson manual is 12 pages in PDF format.