When I went 2X on my 20" folder, I chose 50/34 because I needed more low than I needed high. 34T is the smallest I can go on 110mm BCD. That said, if the BCDs are compatible, if you go 50/34 and miss the 52, you might try putting that on (I'd recommend leaving the chain a couple links longer than you need on 50T at first to allow this possibility) and see if the FD works fine. While modern setups max at 16T difference, there are tons of folks on here that have run 20T difference and shifted fine, and that was with vintage flat rings with no shift pins or ramps on the large ring. What does need to be compatible if swapping out is maintaining the same offset in the ring, whether it is totally flat, or dished to offset the teeth further outboard than the mounts at the spider.