Rembrandt100

I own a 16 ft 2 in Sylvan Backtroller that has a 40hp Johnson on the back of it. I bought it about 12 years ago and it was originally built in 87. The motor has been taken care of and it will troll all day long with no trouble. The problem is with the age of the motor. In the spring of 2013 the push button on the starter would not work and I tried everywhere to find a new one with no luck. Johnson does not have a replacement and it seems no one else does either. I am thinking that some other parts will not be available sooner than later. The other thing that always bothered me was that this motor does not have a charging system to refill the battries. It used to bother me that some times I had to charge the battries before I could take it out. The last trip there was not a good one that ended with someone hitting my truck in a parking lot and leaving. I was not a happy camper and I took it out on the boat. I brought it home and parked it, covered it, but did not change the oil in the lower end. I am considering getting a 4 stroke Merc in the spring. Some one mentioned that the the 4 stroke is not as strong as a 2 stroke. What they said was it would take 50 hp 4 stroke to equal the 40 hp 2 stroke. My concern is not the speed but the how the boat will handle. So has anyone switched a motor like that? Sorry about the long rant but I think I am just trying to justify spending the money in my own head. Dave