You won't need a piston stop. When assembling an F series just install the piston and rod to the crank prior to pushing the piston into the cylinder. This will prevent the piston from traveling too far down in the jug and possibly locking itself into the cylinder.
In regards to converting the plastic carb to a metal Walbro. Yes you can do this fairly easily. Just use the parts catalogues for your 10518 and a 1993 model 10515 to determine what parts will be required. Should just be a carb, air box and throttle cable from a 10515. I converted a 1997 F series model 10401 to a 1993 model 10400 and those parts listed above were pretty much all I needed except maybe some hardware and an optional throttle/choke decal.
In regards to converting the plastic carb to a metal Walbro. Yes you can do this fairly easily. Just use the parts catalogues for your 10518 and a 1993 model 10515 to determine what parts will be required. Should just be a carb, air box and throttle cable from a 10515. I converted a 1997 F series model 10401 to a 1993 model 10400 and those parts listed above were pretty much all I needed except maybe some hardware and an optional throttle/choke decal.
