When we did the mini motards in Brisbane we bought silenced pipes from small wheel deals that got under the max db allowable for lakeside. Only thing though they played havoc with tuning for a bit so have to adjust jets to suit
The yx 160 and the GPX are different motors, the GPX 160 is otherwise known as the 160ho,155z and are also known as GPX and ZONGSHEN motors, the 155z and 160ho are both 155.7cm3 and the yx 160 is actually 161cm3 i believe, but give me a 155z/160ho before the yx. Clearances inlet .003 and exhaust .004
Just bear in mind the differences in power delivery... It might turn out that a 4 stroke will deliver the power that you need where a 2t might be a handful in certain situations. What are you planning on doing with it?