Gilson went out of business quite a few years ago.
Engines will have a date code amongst their various numbers.
Briggs will have a Model, Type & Code. If it's a Briggs AND it's the original engine, you can determine the approx. age of the tiller. (engine will probably be a "few" months older)
Code is YYMMDDzz (ignore the zz, it's denotes plant/line/shift info)
I would assume it's a Briggs 13xx2 Model

The code on the engine is #81091707. I take it the tiller was made on Sept. 17, 1981.Does that sound right? It's a type 085001 & the model # 13020.

I'd bet the Model is 130202? Type is 0850. The "-01" is typically not used.
You have the date correct. Probably a 1982 tiller, IF it's the original engine.
You can download operators & IPL's from the link below.

Here is a link that might be useful: Link Below

Thanks Bill, you've been a big help.

For it to be locked up IMO it probably got water in it as some point. I would just reengine it with a china knock-0ff for about hundred and a half unless you have parts and the ability to completely overhaul the tiller engine yourself.