The cylinder head is the sticking point with these little engines. Until you make more air move through the ports, the bolt on stuff is limited in it's effectiveness.
On my TTR125 I spent a lot of time on the flow bench reworking the ports in the head, but left the engine stock except for lightening the flywheel , making a new end cap for the stock pipe to eliminate the spark arrestor, and jetting. Mine would easily outrun bikes with cams and pipes but no head work.
The Hot Cam is an excellent choice, but it works much better with a modified cylinder head.
If you have limited funds spend it on a Frank Nye - Outlaw Series megaphone pipe. It's the best bang for the buck on a stock engine. Unlike most of the chrome plated crap pipes out there, this pipe works.
http://www.xr100.com/outlaw_series_exhaust_pipes.htm