Need help identifying a harmonic balncer

Hello, I have a 1989 Jeep YJ, which is not stock. I refer to it as a Frankenjeep- it is made up of all sorts of parts from different years.. The Harmonic Balancer has fallen off and I am trying to replace it. Problem being I cannot figure out what motor I have in it to replace the balancer. Every auto parts store I have gone to cannot seem to find the right part either without knowing the year and make of the motor. Which brings me to my next problem, the numbers located on the block do not match any known motor made. I do know its a straight 6 with a 4 barrel. (yes Im a girl lol) I really need to identify this motor. I included some photos hoping someone here will be able to help me, its currently not drivable. Thank you to anyone who takes time to look at the photos and try to help.


You can barely see it, but there is what looks to be an S in front of the numbers. In my research, I did find that could mean it could be a GM or GMC (cant remember) and it was made in Saginaw Michigan.

I see the digits: S99865
90

Please if you have questions to ask of me to help me Email me. Thank you.