How do you remove the cam followers and guides?

[Rupert]

With the list of damage to your original case the replacement looks a better option except for the crankcase mouth problems due to them being from a 350cc engine: B24/25/26.
Having done a few repairs to castings from these late '30's B series engines I've come to realise how fragile they are compared to later BSA engines and how difficult they can be to weld. Partially that's due to how thin the metal is in some places but also because of how old it is with corrosion and contamination effecting how it behaves.
That would have me a bit worried about how easy it would be to successfully do the work on the crankcase mouth which needs doing.
If you can find someone who is capable of welding this stuff then repairing the original casing may be just as feasible as modifying the replacement?
Not sure about the cam spindle oil grooves, haven't seen enough to know whether there should be one pattern of oil grooves or another. Can't disagree with your suggestion that a groove in one direction only will tend to push oil in one direction. I'm tempted to guess that one direction grooves were changed to two direction grooves, same as the later M and B cam spendles are.
In case it's of interest Ive recently replaced the cam spindles on a '39 350 engine I'm doing. I used the standard cam spindles from M and post war B engines and modified them as the oil needs to be fed in from the other end.