The Happy Time expansion pipe from SBP bolted directly onto my GP460 engine. The engine is mounted upside-down, so there was enough room to install the pipe above the engine. The boomerang-shaped expansion chamber is mounted where the top tube meets the seat tube. Then it takes a 90-degree bend with exhaust hose along the chain stay and ends with an SBP lawnmower-type muffler.
This pipe is EXTREMELY quiet. In fact, it is much more quiet than the stock muffler. Its round exhaust flange bolts directly to the 460's square exhaust port. Port matching here might make more power.
More than likely, the HT pipe can be adapted to the Tanaka engines to quiet them down and extract more hp. It would be a simple matter of matching the larger exhaust flange to the expansion pipe.
BTW, this HT pipe adds more hp and does not seem to restrain high rpm.
ANNND, it's cheap.