$15,000 if the fuel tank is full.
Seriously, though, it's impossible to give a price sight unseen, as value depends on a lot of factors. Take 10 minutes to do a search of the archives here on YT and you'll see what most of them are.
If I had to make an offer based on what you told us in your posting, I'd offer $600 or $800.
As for it running 25 years ago, that's one of the classic statements people make when something old and mechanical comes up for sale. ("Ran when parked.") Anybody who's ever worked with old machinery knows that a lot can happen in 25 years... Parts rust, seals/gaskets dry out, freeze damage occurs, mice and other rodents either live inside radiators/fuel tanks/crankcases, or else use them for a nut/acorn repository, stuff like that...
If you post a picture we'll be able to give you a little bit better idea of what you might expect, price-wise.