Here we go again:: I just took my 52 out to the shop and tried it in a (modelfly) mount and it fit super. I then took a look at how it would fit in the Beaver and as far as I can see it should be an easy and neat mount. You may have to cut a little out of the cowl where the head of the engine comes out, but not much. Mind you all I did was set the engine as close as I could but it looked good.
You will probably want to turn the carb over so the needle comes out the top, by doing that it will also put the throttle arm in a little better position. The carb will end up fitting all the way back on the mount in order to get a good monting.
I only glanced at how the muffler will fit but that also looks like a neat fit with little to do.
With a 52 on the Beaver you may get accused of having an electric Beaver. Mine with the 46 inside that big round cowl is Super quiet and when I idle back you can hardly hear it and that's with a 2 bladed prop, if you went with a 3 bladed it would probably be even quiter.
You also ask why I would go with a certain engine. As I said I have 46"s in both mine but I have 52"s in both of my AEROBAT's and I like the sound and the throttle response of the 52's, it seems smoother and more precise, probably just me but that's my opinion. Don't get me wrong, they all do a great job, I just like 4 strokes.
I hope this helps you and please do let us know what you decide.
By the way, when assembling the plastic planes make sure you rough up any area you will be glueing, I use a hobby knife and SCORE all those areas to make sure I get a good bond. ESPECIALLY on the screw post that go into the float, I score the post and put plenty of glue in the hole as it is going into foam and I want it to spread out inside to make a better bond.

Have fun and ENJOY !!! RED