The copper pipe idea I think was when using 2 Victa decomps or if there wasn't enough room for the decomp in the head.
The Royal Enfield ,I've seen 2 types ,one manual and one cable operated that released compression into the exhaust port, You shouldn't have any trouble screwing the decomp straight into the head.
I see there are Harley decomps that are built into the spark plug hole but a little expensive.
Lot of different bike ones out there that are cheap ,I guess the only other concern is if the decomp closes to late after compression and sucks in dirt.
