What year is your PE? Only the top of the cylinder is the same part on RS and PE and only for the 1980-1984. The cylinder itself is different for the RS and PE. I believe the RS has a lower exhaust port, maybe the pipe could be modified but it seems like a lot of trouble. Check with Brenden at Bike Bone Yard (
http://www.bikeboneyard.com/) for a real PE cylinder or wait till one comes up on ebay. 1980-1984 PE175 all use the same head though.
Here's my 1982 RS175:
http://www.bikepics.com/members/mshade/82pe175/