How much end play are we talking about, the start shaft will have a little, but not a ton.
It sounds like maybe the start shaft or the nub on the crank **might** be worn.
I would pull the engines backplate off and inspect the start shaft and crank end if you had to hold the start shaft in to get it engage the crank.
Do you still have the old OWB? If you do, slosh it around in denatured alcohol, rubbing alcohol if that's all you have, and blow it dry.
Thoroughly clean the shaft with alcohol and let it dry. Now reassemble them and see if the bearing only turns one way.
If you tossed the original bearing, do the same procedure I just mentioned, with the new one. When it's on the shaft, reinstall the pull start and test it while holding the engine in your hand. If it's working correctly, install it back in the truck.
After doing some digging, it looks like the HPI F4.6 engine's starter shaft is supposed to get pushed into the crank to engage it.
I assume here that the pull start is supposed to push the start shaft in when you pull the cord.
I think it's going to end up being the pull start.
Below is what the F4.6 start shaft looks like. http://www.dollarhobbyz.com/hpi/tro...tar-f4-6-backplate-one-way-starter-shaft.html
I do not have a HPI pull start to look at but that's where I would look at.
I just realized why it's not working. You're missing a part. In the HPI setup there's a pin on a spring that locks into that groove to grab the crank when you start it.