i am using a 18cc weed eater for a rc boat motor and i need to know how to make the shaft work for a boat. and what i need to get it turning to get the engine going because i broke the pull starter?

Couldn't you just fix the pull starter?

would a recoil starter from an rc car work or be easily modifiable?

You need to fix the pull starter!

A car engine pull starter (.21 size) will NOT work.

Some of the lawn shops will fix you up with a good secondhand one, or maybe repair the old one cheap (if it's not too bad).