Well, the problem is that the VIN doesn't help since the motor is not the same year as the car. I know that the car/chassis is an 89 but I think the motor is out of a 90-91 but I'm not positive and the guy that built the car is no longer with us. Can you tell by a number on the motor itself what year it was made?
There should be a casting number on the block somewhere, but I'm not sure where on 2.3s or how to decode it for a 2.3, if you look on the cylinder head (probably under the valve cover) you might find a part number, it'll start with a letter followed by a number. That'll tell you what year the part in question (in this case, the cylinder head) first came into use by Ford.
The first letter defines decade: C=60's, D=70's, E=80's, F=90's
The number that follows tells you what year in that decade.
So if, for example, you found a part number on you cylinder head that started out with "E7" as it's first four digits, that would be a part that started production in 1987.
I know that's not an exact answer for "which year" but it should get you in the ballpark, as the cylinder head is where many of the significant changes happened in these engines from 1988 to 1997.