toosidewaystoby

I have the opportunity to buy an fc or an fb shell. I also have enough to invest in a 20b rotary or a 13b rew (as the engines itself). I know for a fact I can do most of the mounting and sway by myself, the most difficult part wil be the wiring. My question is, in the end will it be cheaper to throw a 20b into a fc or fb, or will it be cheaper to throw a 13b rew in an fb or fc? Right now I daily a 13b fc, but I'm interested in building another rotary. So like I stated before, which one of thes swaps will be the easiest and least expensive. (and yes, I know it will be expensive). But I'm really just looking for exact details and modifications, no building of the engine or anything like that, just a simple swap.