Hi Charlie
Can i assume the purpose of the exercise is to install the larger dia gear into your belt drive pulley ?. If this is the case you are still going to need the drive key.
As m-c has mentioned the chances are you should be able to machine the gears OK with a machined sleeve, an option you could try is to use the original keyed pinion and remove the geared portion, bore the new gear to suit but leave a slight clearance between OD & ID and silver solder the new gear onto the original sleeve, so no need to machine the pin. If you fancy this option, to bore the new gear make the sleeve with a spigot ( ie like a top hat) this will be pressed hard up to the chuck jaws, size the OD of the sleeve 4mm greater than the gear OD, machine the bore of the sleeve but leave a step in the back of the bore to locate the gear on do not remove the sleeve from the chuck or you will loose concentricity.push the gear into the sleeve using tailstock and then tighten the chuck to deform the sleeve and grip the gear.
Hope that's of some help.
Mike