Given the cost to rebuild a 2004 model, $2,700.00+, and calculating the mileage 3,600 hrs X 25 mph = 90,000 miles, your mechanic is right on the money.
Too expensive to fix.
He is probably looking at the entire vehicle and knows more than any poster here just what shape the rest of this machine is real in.
You said they did all the maintenance, my guess is they now best.
Getting it fixed and running would just be the start to many repair bills later for all the other worn parts.
Easy to flame a good mechanic when they give truthful bad news.
Next steps, if you can afford the money, have him tear some of it apart to have a good assessment done of the repairs. Even if it's limited to say $100 to see if the head is cracked.
This is possibly a good buy for the DIY guy who likes to tinker and can afford the parts and do the rest himself.