Fitting new hammer to Pro Turner

[Judders]

Anyone give me some guidance on replacing a hammer in my old Pro Turner underpinner?
I know the basic principles of dismantling the push rod by sliding off the guide block by way of the slot,
this removes the hammer assembly I think you loosen the grub screws to withdraw the hammer together with the spring. Not sure where the locking pin is for the hammer. I think it slides up a tube, to locate on the top plate, and I gather this is a bit awkward to feed through! I know how important it is that the block has to be dead level finally for the push rod to slide through to avoid it jamming. The hammer no longer protudes even 1mm at the top now and has not been replaced for over twelve months now. Any tips would be appreciated, thanks.