You can get a bit creative with the speaker's physical dimensions. Just keep in mind the old RS4/5 audio amp can drive 1.5W and needs to see at least 8ohms for a speaker load. I really doubt you can hurt one of those Fat Boys though.
Some of the newer 40mm 16ohm Baby Fat Boys are OK with the matching dual speaker enclosures.
I have been hunting and collecting speakers for my upgrades and mods. Here's a nice oval one I got from eBay that works well: