The lion's share of the effort and expense in that modification is going to be fabricating some method to remount all your indicators. Seems to me like the very best possible outcome will be a poorly performing speaker that only plays one side.
You can't get stereo through a single driver without buying a dual-voice coil speaker, and there are very few available in that size. You're still going to be disappointed though, because of the short cone excursion. The distortion you're hearing is from the bass, and a 5" in-tank speaker isn't going to have any bass either.
I see only two real possibilities that will give decent performance without spending a ton of money on a new fairing. The simplest would be some marine-grade enclosed speakers. They come in rectangle boxes around 6x8 and have threaded mounts. You could fabricate a bracket to mount them on your highway bars, or perhaps mount the bracket directly to the top of some hard bags if you have any.
Also, you get what you pay for. $20 Pyle speakers will sound like $20 speakers. These Kicker enclosures
are pretty nice. Your Shark amp would still sound terrible paired with those speakers. You'd need something better like the Soundstream Nano
. They're affordable and perform well.
Actually, that's pretty much the only method that's truly affordable. The next thing that came to mind was modifying one of those DMY trunks. Mine has 2-4" marine speakers mounted in it. If no one was sitting behind me I could also mount a front-firing 6.5" or 8" woofer in there, and then separate sub-enclosures for the small speakers to reduce distortion. That would require a pretty powerful amp though.
Bottom line, if you want big sound you're going to have to put some money into it. I do think the external enclosures with amp would work pretty good, but you're still going to have on the order of $300 into that when you're done.