the bat tray will accomodate a group 1 6v bat, so you need a 12v bat of similar dimensions.
grp1 = 9 1/8 L, 7 1/8 W, 9 3/8 H
thus, just running down the group chart, any of the following will fit within those specs.
21, 22r, 25,26,35,52,54,55,61,62,70-73,75,85,86
I'd like to point out I would not reccomend the 'reversed' terminal unit, .. though you could do it if the cables are long enough to reach without interference., also, the grp 52 is a pretty small battery 5 13/16 width.. likely the narrowest one in t he list etc.., .. probably smaller than the rest. you may actually need a wood block to 'bushing' it in place.. it will have a lower CCA capacity.. but then.. 12v bats on a 6v starter actually work pretty good, low capacity or not.
also.. there are at least 5-6 other batteries that were 1/8-1/4" larger in length or width than the grp1 bat, i did not include those.. but it's likely they would fit as a grp 1 6v bat still has a scoth of play in the bat tray. Don't try to go larger than the Height number as you will have battery post / gas tank issues!
hope that helps some..
soundguy