I'll contribute what I can. I'm no "tire expert", but I have put super singles on a deuce a time or two.
PK-
Those 12.5R20 tires should 13"wide and 40" tall (same height as 900x20). I'm not sure if they will work as duals, but I would run singles anyway. Just flip your hubs and you are set. What are you going to use for rims? The approved rim for the Michelin XL 12.5R20 is 11".
JR-
For stock rims, you are best off with 900x20s or 1100x20s tires. Other sizes "will" fit, but not advisable. 900 rims are 7" wide and 1100 rims are 8" wide. If you can get/make wider rims you can go with options like 12.5R20s (40" tall, same as 900s, take 11"W rim), 14.5R20 (43", same as 1100s, take a 11" or 12" rim), 14.75/80R20 (45" tall, 10" rim), 15.5/80R20 (47" tall 10" rim), or 395/85R20 (47" tall, 10" rim).
These numbers represent specs from Titan Tires (NDCC) and Michelin (Radials). Other options may work. Post up any other questions you might have and I'll try my best.