Without access to new carbon seals I have done the following:

Order from John Crane seals an 87079067 mechanical seal. Then get the impeller machined so that the seal fits and have the machine shop seal it in place as required. The design of the seal is entirely different so it needs to be sealed. Get them also to make up a brass top hat bush to fit the body snugly. The rubbing face will need to be a mirror finish. Let the machine shop remove the old carbon seal so that the face protrusion can be checked against the travel of the new seal spring. The mechanical seal I have requires a bush to these dimensions:

overall height:0.494"
flange height: 0.177"
Major od :1.437"
minor od: 1.045"
id: 0.687"

You will need to check the shaft is not bent and have the impeller checked for run out once assembled to the taper of the shaft and fitted in the bearing, since it will act as a wobble plate and leak if not running true. You will probably need to have the housing clocked to ensure that the mating surface and bore for the top hat are parallel to the body 1/2 mounting face and/or at a right angle to one another. This operation will probably alter bush dimensions so the machine shop will have to measure the existing body and adjust as appropriate.

Having this done is only 1/4 of the required work. The bushes in the end caps will more than likely need to be renewed, that is if the shafts and gears are still serviceable. IIRC there are shims between the end caps and the housing to control the end float of the gears.

Do NOT use silicone sealant instead of a gasket to refit the pump to the engine, just as when fitting a PTO drive you will need to trial fit the pump to ensure that the intermediate gear and the engine timing gear have sensible backlash and adjust the gasket thickness if necessary. If you fail to do this and there is no backlash then the engine timing gears will be chewed up. It is common to find that previous overhauls have surface ground the mating face of the gear housing thus bringing the gears more closely into mesh. The mating surface of the seal and bush will require initial lubrication with grease as would the carbon seal.

This will cost a lot more money than originally thought.