IF the engine is original, go to this link and download the service manual. It will tell you how to date the engine.
Engine is likely a "few" months older than the tractor.

http://www.kohlerengines.com/onlinecatalog/productMatrix.htm

Way i do it is this:
Set it out front with a "For Sale" sign on it. You should have a price figgered out for it, with a high and bottom line on it. Not necessarily on the sign. If and when somebody stops to look at it, if they think the price is good-they'll buy it. If not-well another one will be along in a day or so.
I have good success with that method. I've sold 4 machines this summer--all i had. And, next years stock is coming in. Got 2 since last Sunday! No, not a dealer, just a person who fixes mowers and lawn tractors, for spending money. Rusty J.

Its value is same as you purchase it for in 1998 that wheelhorse 312-8 not junk I hope you got plan for something better but there nothing could be.