Also worth changing the oil in the grearbox - if it's turned to mayonaise, then the case seal(s) weill probably need replaced or more likely the washers behind the filler / drain screws if the previous owner(s) have re-used the old ones umpteen times.
These engines in the early days were notorious for destroying themselves. One to definetly check is the spark advance. The later models were retared by a few degreees and allowed to rev a bit higher compared to the early models. Having said that, as you have a 50 the spark may already not be at +23 degrees or whatever the number was for the early 60. Have a look into your top cyl through the spark hole with a torch - if the "dam" is eroded near the spark, you proably want a bigger jet in your top carb amongst other things. - Give Alex a call at Mitchell's in Renfrew - he can give chapter & verse on what needs changed. (and is pretty quick at getting spares for older engines)
I suspect I may have a pile of spare O- rings & gaskets for your vntage left over from my rebuild (I got the "complete engine" set, and it came with a couple of subtle variants on a gasket for certain joints) if you find you need one, Let me know, worst case the gasket was never modified and I'll have used it already!
PS Crankcase oil seals are pretty cheap - but if you do them, buy the powerhead gasket set as well, there is no quick or easy way of changing them!