maybe its possible for a nitro engine, but its IMPOSSIBLE with a weed wacker!
save your time and trouble and just buy the recoil part, from expiriance, i have put recoils on lawn mowers before and after an hour you get it togather, then you pull the rope it works, you think "fixed!" but then you go and start it again, shwing! it pops out again.[:@]

you can get a bump starter or bump box, they are a pain in the ***** though, but i had two of the same engines before, ofna red heads, one with pull start one without, the one without gave me better responce and another 3k rpms which is enough for a few mph, and the one with pull start lost responce time and was 3k rpms short of the non pull start.

one of the reasons because of this is the bearing causes friction and extra rotating mass, then another one is that you get some blow by of the back plate reducing proformance and pressure. puls if the gasket fails, you lose valuble oil and it makes a ugly mess.