Noel
To remove the pinion from the gearbox is a swine of a job at the best of times. Before you try and do this, see if there's any play fore/aft actually within the shaft itself. If it's good, leave it alone. Trust me, you'll only end up breaking something if you try and go deeper without the correct methods or tools.
If on the other hand it is too far gone, why bother going to all the hassle of rebuilding a tired old gearbox. There's plenty of these around on ebay or elsewhere.
(or somewhere else wink).
But you're right. The crown wheel is a gonner
On another note
How i remove the drive shaft is like this.
With the pump housing removed i then clamp the square drive shaft into a vice, good and tight. (probably a good time to order a new impeller for the next bit)
Heat the bugger up around the base of the driveshaft (you'll probably end up melting the impeller as you do this but don't worry, these are easy enough to replace later on)
Then with a rubber or copper/hide mallet and a lump of hardwood, belt the bejeezus out of the top of the gearcase (not too heavy though as you can crack the casting if you're not careful)) and it should start to come off slowly. It might take several attempts to get it to move, but stick with it and it will go. Please DO NOT strike directly onto the cast aluminium with a hammer, use a lump of hardwood.
And if you're really lucky (provided you've removed all the bolts before hand) the impeller shaft might come out at the same time.
Heat, and LOTS OF IT. Don't be scared
Jon