Will the lower unit off of a 1985 Mercury black max fit a 1996 Mariner 200?

Sorry it is a 175hp black max.

Yes.

Thank you Mr. Pender. I wasn't for sure.

I think the gear ratio will be different...If not mistaken (which is very possible) I believe the 175 is 1.78 and the 150 is 1.85

Not a big deal, but something to be aware of.

Most 150hp came with a 2:1 and the rest had 1:87 to 1. You can mark your prop shaft and turn the fly wheel and count the dive shaft revelution to see what you have. Pull your spark plugs so your not working aginst compression. You might also pull the kill switch.