Saw a Leo Vinci fitted to an Xmoto in bahnstormers on saturday, It's road legal but not very quiet from what i understand.
Remus do a system for it, but as i understand it, it's purely for off road use, and isn't road legal.
Bos also do a system, and this comes highly rated.
I think Bahnstormer have an Akra system on Lee Walters race bike.
I'm planning on giving the Bos a look, but theres currently to much other stuff that needs fitting to the bike first....