You could try the Rotostart that they are fitting on the RTR kits.I use one and have no problems with it except that they use a 7.2 volt battery and thats a pain to charge up often and when the battery is low it won't turn the engine over fast enough to fire it up.
Or theres the good old starter box,other than them I don't know what to tell you to use.
Are you over pulling the pull starts more than about 12-15 inches? that could be your pullstart problems or you might of put the oneway bearing in the wrong way when you where fitting them,if you said you are on your 3rd!
Hope that helps[]