It's one of those things that you have to look at from both ends. One end is, what is it worth based on parts it has, and the money that has been put into it. The other end, is what is it going to cost to get it fully functional. Then, the real value ends up somewhere in the middle.
For instance, there's been $1,000 put into the engine. New front tires and tubes, maybe $150. A clean 1655 with deck and blade, is worth another 700 - 900 depending on condition of the plastics, the steering play/bind, how many hours are on it, condition of the pulleys on the deck and splined shafts, etc. Add all that up, and you're pushing $2,000!
From the other end, a buyer would be looking at buying a battery ($100 for a good sized automotive), two rear tires ($200), a three point hitch and cylinder presumably ($300), a seat ($75), rear PTO parts (??), some paint and cosmetics (??), etc. Add this up, and you're looking at investing another $700 - 1000 to get a fully functioning unit!
So, I don't know. Somewhere between $400 and $1,100 seems fair :)