What is the condition of the rest of the car?
Neil.
As Neil has said , what is the rest of the car like before you make any hasty decisions
Craig 
excellent condition, a have some pics if anyone wants to see, but you will have to send me your email address if you want to see the pics, but when i say bin it what i mean is bin the idea of a rebuild, and just px the gto in on something, shame as its a 1991 twin turbo manual, just had a new stage 2 clutch fitted from evil rob, done about 400 miles, plus the extras its got hks exhaust, hks fcd, elec boost controller, tein suspension ect
A gto with a knackered engine wont be worth a great deal , that said if you like the car why not rebuild the engine ?
A few ways to do this either buy a running lump to keep it on the road while you rebuild yours or buy another engine to rebuild .
I always prefer the first option depending how long you have had it and how much you know the recent history
Craig