RE: Kits or scratch plans?
I'd choose the OS .21 RG-M and the standard size Wild Thing over a "biggie sizing" it for the .32. The .32 SX is a pretty good sport airplane engine, but in marine version, it's specs are anemic when compared to the .21 RG-M. Also, it would be really good to look into buying an electric starter rather than going with the pull start version of either engine. The pull starters add weight, $, extra maintenance, and rob a bit of power. You can get a starter and 12V battery for not much more than the extra cost of the pull start.