There is a good guide to working on the gearbox here …
You don’t need to go all the way in to change the return spring, it is just behind the rear cover. I did do the reshimming thing a few years ago but didn’t notice much difference.
https://www.thisoldtractor.com/guzzitech.dk/dk_da_complex-technical_gearbox-rebuilding-pictures.htm
https://www.thisoldtractor.com/guzzitech.dk/dk_da_complex-technical_gearbox-shimming-pictures.htm
Yes, once I had cleaned up the pinion there were obvious wear ridges at the halfway point👍🏻
I had a look at the pushrod and couldn’t see any heat related blueing to suggest use with a lack of clearance. No burring at the end and it is straight when rolled over a glass plate. £50 odd from Gutsibits so may be tempted to reuse🤣 I will replace the input shaft old seal, the recommended 6 pushrod seals and the bigger o ring for the piston at the lever end.
Cheers Don. Very helpful. I will renew the spring and check out This Old Tractor.
Hi, i have built several of theses and just finished the T3 Cali shown on the for sale page now sold. I have a comprehensive pic file of stripdown and rebuild as well as reams of technical literature so can tell you exactly what you need tool wise, parts wise as well as working methods. While your in there check everything else out drive train, swingarm pinions, UJ, rear wheel and order new bolts fixings and schnorr washers for the ring gear flywheel and bevel box. .Then there is the correct way to fit the frame rails swingarm and drive box, it goes on ,things you don’t think of but if your keeping the bike it’s worth it. It’s a balance of cost at the end of the day though but if you can afford to, do everything. Any help you need just email me ,Neil neil@owcodisplays.com PS i worked with Greg Bender on several things and bought all electrics from him during the rebuild and yes he is a top bloke
My only problem with taking the rear cover off to do the spring was that one of the shafts came with it! But not too big a deal to put it all back in again. It’s a lot easier to see what needs to happen when it’s in front of you than to describe it in writing. My bike is a loop-frame so I don’t know about access to the eccentric adjuster on a Tonti bike, but I would suggest checking that the new spring hasn’t messed up the up/down detent adjustment before putting it all back together.
Oh the joys of buying second-hand! 
There are often surprises - previous owner bodge-ups, mods and ham-fisted repairs - it may look a bit like a money pit at the mo but to be fair this is typical kind of wear, and once it’s sorted, it’s sorted! 
Cheers Neil, if I get stuck or need advice on the final drive I’ll be in touch👍🏻
I’ll do as little as needed and hopefully it’ll just be the spring.
To be fair I’ve done a few thousand miles last year on her. All she needed was oils, a filter renewal, a new battery and a reset of the points and timing and she’s been great until the clutch failed to disengage. I did a 550 mile day on her, and 1200 miles in three days without a stutter at high speeds and negligible oil use so reckon she’s pretty sound. I’ll put the £250/300 of parts into her and hopefully enjoy this summer on her again.
Gutsibits got back to me and it’s approximately a 3 week wait for the clutch kit.