Modern Vespa

Ok, so finally have rounded up the majority of parts to rebuild my engine. I had a spare hour today so I put a new bearing in the new cush drive cluster and fitted to new case. I then fitted the gear cluster which was a new(er) second hand drive shaft with 2 of my original gears and 2 newer less worn gears, new cruciform and rod. Once I had hammered the driveshaft through the new rear axle bearing (yes with a drift and not smashing the end of the rod) I noticed there was a small amount of movement with the rear axle and it didn't seem like the two sets of gears were meshing properly. I would have to push the rear axle forwards a couple of mm to have them mesh without slipping at some points. I then put the fly side case on temporarily to see if this would seat the rear axle properly through both sets of bearings but I could still get some of the gears to slip when spinning the Cush drive and applying resistance to the axle spline.
I noticed I could pull the rear axle out of the engine case as it wasn't seated firmly inside the bearing.
Would this mean I have a mismatched set gear clusters and the newer axle may be more worn than I thought meaning its not tight in the rear bearing?

Which bits are new and which are new, and have any of the gears been together before?

New Christmas tree assembly from SIP, original 2nd & 4th gear, good used 1st, 2nd & driveshaft. Used parts sourced by Mark Carter.