Hi, since those are both Inline Sixes, given their model years neither would have come with a voltage-regulator-type of rectifier. So the 115hp must have had a retrofit.
If you do indeed have a true voltage regulator on the 115, it should transfer over to the 90hp with no issues. It's not gonna affect what I assume is the ADI on the 90hp. All you're gonna have on either powerhead are 2 yellow stator wires coming out from under the flywheel. Those connect to either yellow lead on the regulator.
The red +12V wire connects to either one or two red wires on the V. Reg, depending on the style. If there are 2 red wires, just tie them together and connect that red wire in the motor harness.
The black wire coming out of the V. Reg should go to a good engine ground. You'll probably need to build a mounting plate; a hunk of aluminum would make a good heat sink for the regulator.
I see lots of V. Regs on eBay, some are very cheap Chinesium but should still work. The real Merc V. Regs are shockingly expensive now.
Here's an example of an El-Cheapo V. Reg on eBay that probably would do the job:
https://www. ebay. com /itm/354432749880
This site still parses eBay links, so just copy and remove the spaces. Or copy the listing number and search eBay for that, it'll pop right up.
Hope that helps.............ed