Im not familiar with whatever engine you have but I assume its the stock engine.
It can be done, but you would need the back plate off of an engine that is non pullstart.

Like I say I cant remember the internals of that engine (Dont have it anymore) but if the crank has the nub on it that would have to be filed down so the non pullstart backplate will fit.

Are you wasting your time and effort? Yes! In my opinion you are. Removing the pullstart will not give you much difference in RPM. If you throw a lightwieght flywheel on there in addition to the removal of the pull start...maybe a slight notice of increase.

You are better off just using the starter box and leave the pull start on there. Then once you wear that engine out get a non pull start engine.

Dbow