Lister Junior A - exhaust valve and big end shims

So, I've been working on my recently acquired spec 25 A Type.

I'm in need of a good exhaust valve if anyone has one? The valves and guides are in OK condition with the exception that the exhaust valve is slightly bent so needs replacing.

Secondly, what's the procedure for reshiming the big end cap and when is it necessary? The journal and shells seem OK but there is a very small amount of play evident when rocking the conrod. Currently there are a couple of brass shims fitted each side of the cap.