Model 'B' - engine reassembly advice needed

[scottsscoot]

Hi, has anyone here rebuilt a Model B engine?
I may have done a foolish thing & I’m now in need of advice…
While resurrecting a ‘B’ heap I took off the magneto housing not realising it was also the left-hand crankcase half. My first thought was to tap it back on with a rubber mallet, but I doubt the bolt-up crank might not take the abuse. Can someone tell me the right way of doing the job?
TIA

[jbcollier]

Can't find a B or C service station manual. Here's a D/LD one that may help:

https://www.lambrettajourney.co.uk/file ... manual.pdf