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Sailor, it's obviously your decision, but if you generally like your Johnson 50, making an effort to get it working properly would seem to make sense. It's not a sure thing that you'll be able to get it working, but if you can, you save a ton of money.

You're looking at 3 steps:

1) Fix whatever is wrong with your motor. While a decarb won't hurt anything, it sounds to me that it may be time to rebuild the carbs. This is not a huge deal - many consider it to be a routine maintenance item that you need to do every few years. If you are handy, you can get the factory service manual and tackle it yourself, otherwise it will be several hundred dollars for a mechanic to do it. The parts are about $35 per carb.

2) Lift the motor. I had a Johnson 70 on my 15 before I got the F70. The motor was all the way down when I bought it, and I ended up raising it two holes. It was not a massive difference in top speed, but the Johnson 70 had power to spare - my guess is it would make more difference with your 50. Raising the motor did significantly improve the handling - lightened up the steering quite a bit. You can do this job yourself, if you are so inclined. Also note that if you end up going with the F70, you'll need to do this job anyway as the F70 cannot be mounted with the blind holes.

3) Get a new prop - this may or may not be needed depending on the results of 1 & 2. You have not specified what prop you have on there now, but assuming you don't have one already, a good quality, modern, stainless steel prop will really make the boat come alive. It may take a couple of tries to find the perfect one for your situation, but if you are looking to keep the boat for a long time, it's worth it.

So my recommendation would be to do 1 & 2, and wait and see on #3.

For reference, the switch from powerhead rating to prop rating occurred in 1986, well in advance of the manufacturing date of your motor.

Tim