lol any external filter will work, you can get a primer filter, that primes engine by pumping,(crap if ya ask me) or get a small durbro one off ebay 1.99 does its job!
also when you have replaced the pull starter have you noticed any oil residue around that area? doe's your old pull chords look like they were soaked by oil? the reason i ask is , i have seen a few .21 ttr that have a dodgy back plate bush, (were the shaft comes out the back)
take a look thu the forum and look for a how to find air leaks how to, quite simple really, take engine off chassis, block carb and exhaust hole's , leave fuel line connected to carb, mix up some fairy liquid in water nice n bubbly, (like a aero, mint pref) now with a small paint brush dab some of the fairy solution arount the back plate, carb, and blow down the fuel line, any air leaks will result in little bubbles in the solution you jave just applied!also put some around the hsn and lsn, a dodgy seal here and youll never hold a tune!