I would be very concerned about the age of the belt - it's 23 years old. I think that when evaluating rubber components, you have to consider both calender time and mileage... what would you think about riding around on 23 year old tires, even if they had sufficient tread left?
Having said that, I don't think that changing the timing belt will make much difference to the resale value of your bike. At that age, the selling price of the bike has more or less stabilized, and I think that buyers would expect that the belt has been changed at some point in time. If it has not been changed, you might get less for the bike, but I doubt that you will recover the full cost of changing the belt when you sell it.
The real question (to me, anyway) is whether or not you want to "roll the dice" for another year, and risk the consequences if the belt fails. Besides the rider safety concerns, if the belt fails, the bike will be scrap, all you will be able to do is just part it out, which will probably be a lot more time-consuming than changing the belt.
So, I'd vote for changing it, but primarily for safety considerations during your next (& last) 6,000 miles of riding, not for any benefit to resale value.
Michael