I found the pipe, but also a tote with a bunch of parts that came with the engine including what looks to be an RSC liquid cooled cylinder, head, waterpump and a bunch of pistons and rings and what not. I believe the pipe was made for this cylinder as it slips nicely onto the spigot
I doubt very much if that pipe will fit an aircooled bike without a lot of modification as the spigot for the exhaust is huge on the water cooled cylinder. Its way, way bigger than my 350 even

I don't know enough about these bikes to know if the cylinder helps the motor or is just sort of a junky, gimmicky thing to add watercooling for the "coolness factor". I'll ask on the UK forum.