The FMF pipes are overrated. They really don't do much. And as far as the air filter if the one you have now is in good shape just leave it. A new air filter will just cost money, you wont notice anything. You shouldn't have to rejet with an FMF pipe but I recommend the Pro Circut T-4, it is a great pipe and will really boost performance.
A shop that does a lot of my engine work designs, builds and sells their own cams for the Xr70. The cam will do more for the bike than any pipe could ever do (and the cam is cheaper than the pipe anyway!).
The shop is Aurora Cycle. Give them a call (Rob is my contact there) 503-678-2200.