SS gave you some good info, but now comes the hard part of locating the jets. Most likely, local shops won't have the parts or won't be able to order them, count yourself lucky is they can get them for you. I had to find some NLA parts for my 89 Force (Chrysler) and it took a couple hours of scouring the internet, but I did find a shop that had them, pure luck I thought...
But as Frank said, that combo even in perfect high altitude tune, very well may not deliver the desired performance with 4 adults onboard. Leaner jets and even a lower pitched prop are the best you can do. A tiny tach will help select the prop pitch, though it may very well be a waste of money if the limited HP isn't going to work.
Frank, is there a lower gear ratio lower unit available for these outboards? I know I've seen talk of some outboards having different ratios due to year, HP or usage. Probably not, but I thought I'd ask as I know this is one way to compensate for high altitude,,, lower gear ratio.