1996 Ford Taurus computer question

Engine Mechanical problem
1996 Ford Taurus 6 cyl Front Wheel Drive Automatic

After having a head gasket replacement done, it now has a miss in the #1 cylinder according to a timing light. It has been put on a borrowed computer and it reads several codes, one being a misfire and some others that haven't been mentioned
what they are. The guy doing the work strongly feels that it needs a new computer. Like I said, it runs with a miss and after running for a little while, it starts backwashing thru the reservoir. It was doing this before the head gasket removal. In your opinion, is ordering a new computer really what it needs? The mechanic says the misfire is causing the overheating which is causing the backwash of the cooling system. Please help!