I used to build quite a few 064's using the 056 Mag 2 cylinder and piston assembly. They are not really too hard to adapt the Mag 2 stuff onto the 064,but the catch was fitting a correct piston bearing... The 064 bearing doesn't fit, and the 056 bearing is too wide for the 064 connecting rod. These saws were really popular with fallers in my area. The ones I built for fallers I did not bother to relocate the spark plug hole of the 056 cyl. to the rear like the 064's (The 056 plug sticks straight up from the center of the cyl.),so I cut a hole in the top of the top cover for the spark plug to stick out.
It really isn't a too hard swap,it used to take me a full day at work (about 8 hours) to finish installing the Mag 2 cyl. on an 064. The fallers loved them... a light 100cc saw (98.6cc), and they fit the 100cc competition saw class nicely.
I can explain the install in detail,but it's quite easy.
The bottom ends holds up fine, none of the ones I built ever blew, and some were for fallers, run hard day after day. The 056 piston was considerably heavier than the 064 piston, and I too originally questioned how long they'd last. A lightweight wrist pin helps.
As a home or work saw they are easy to build. As for a race saw, trying to stuff a big race carb in the airbox and getting it to work and look stock with the air filter cover fitting intact was quite difficult.
The nice thing about the 056 Mag 2 to 064 swap is you get a full 100cc (98.6) saw in the small lightweight 064 package,without using inferior aftermarket parts. And the 064 didn't have a rev limiter (Most),or a carb with a fixed high speed mixture.
To make the Mag2 cyl. fit you must file-slot the cyl. bolt-on holes inward slightly(use an 064 base gasket as a template),and I would tap out the 064's cyl. bolt holes out from 5mm to 6mm, like the Mag2 (or 066) uses.You could maybe skip this step and use the 064's 5mm screws,but I always made them 6mm. Then you have to trim a few cyl. fins so the 064 top cover would fit on. To use the 064 muffler you had to heat it with a torch and bend it downwards and then it would bolt on like stock.
If used as a general purpose or work saw,I would bore out the stock Walbro WJ carb and use it.
I built and sold quite of few of these brand new to fallers,and most were modified too.(Some porting,muffler mod.,etc.)None of them blew up or failed that I know of.
Anyone considering building one can contact me,and I'll give you the details and suggestions to build a good one.