For $50 you can get a Stebel Nautilus that is pretty darn loud and easily fits a Buddy- https://www.amazon.com/Stebel-Nautilus- ... B003ET0JJQ
I believe this is what a lot of the special edition Buddy got, and all the 150cc(?) I would like to get one of these at some point.
If you want to get crazy you could buy a front rack and a 12v compressor and air horns. Something like this - https://www.amazon.com/dp/B0B1PYNCWZ/re ... rd_plhdr=t
then mount the horns on the rack and mount the compressor by the original horn.