Like Franque said, the TW200 countershaft is too long, so it's not a candidate. The XT225 or the TT-R230 is a good fit. You could also pop in a Yamaha Exciter 185 engine, although that's a five speed and has less power.
Bottom line is that the Sierra was designed for a Yamaha engine, not a Honda engine. Yes, you can fit a Lifan Honda clone engine in the space with some welding and grinding, but it's not a bolt-in deal. The original engine in your Sierra was a Zongshen ZS200GY-2, which is a near-perfect Yamaha clone. There is a ton of good info here:
http://www.chinariders.net/showthread.php?t=6051