On a different tack. If it is only $300 for
exhaust and 1.5 hrs to hook up engine that's not too much $$$$ wise. Up your offer by the amount to cover the yard doing the work and put the responsibility on the CO to get the work done. Make the entire deal pending survey. If he is telling the truth, he shouldn't have any trouble with the deal. This is a standard practice when buying a home, but often overlooked in other transactions.