Johnson and Evinrude never went out of business. The brand is owned by BRP. BRP chose to discontinue production but they still own the brand and the rights to manufacture all parts.
In 1990 Johnson and Evinrude were owned by omc. The late ’80s and early 90s in my opinion were the absolute pinnacle of OMC quality and design.
Common parts will be available for these OMC outboards practically forever. The brand was selling well in this time period. And there are millions upon millions of them out there.
The other thing to look at is that there have long been other aftermarket manufacturers making OMC parts. In fact almost all common parts I use on these outboards are actually aftermarkets not OEM parts. Relatively speaking outboards in general rack up very few hours so there will be OMC outboards running well into the next century.
Do your maintenance. Store the outboard properly. Use good fuel and run the tank as close to dry as possible between phillips. Use quality oil. Do all these things and that outboard will probably be still running just as well when it’s 50 as it is today.