Re: will a lower unit from a 1972 85 HP Chrysler fit>>>
Yes, it will fit, but your 90 will not perform quite as well---The 90 has 2 to 1 gear ratio and the 85 has a 1.78 to 1 ratio. If it is just gearcase damage, transplant the 90 gears into the 85 case. Get a manual and be sure they are shimmed properly--otherwise the pinion and forward gears will self destruct.
If the gears themselves are damaged, then you will have to live with about 2-4 mph slower top speed.
