I have only rebuilt about half a dozen MV engines, so I don't claim to be an expert, but they are very similar to the air-cooled motor. I must have built fifty of those.
Here's my two-bob's worth;
Cylinder heads and valve gear are the hardest working parts of the engine, and although it is feasible to recondition them, new heads are definitely more reliable in the long run.
This is because few machine shops are set up with the correct jig and step-drill to drill out the valve guides - most of them just drive out the guides with an air-chisel, and remove some of the aluminium at the same time. The new guides, when fitted, then come loose with use.
I have used AMC heads on all of the water-cooled engines without any problems.
They are less prone to cracking than the genuine VW heads.
A friend of mine who runs a cylinder head shop tells me that AMC make heads for diesel trucks, and they are a good product.
However, I have seen one used AMC VW head that had obviously had one of the exhaust valves replaced, as it did not match the other three. This suggests that the valve must have stretched or burnt and was replaced before it failed catastrophically.
Having read the blogs, I decided to err on the side of caution with my next rebuild. I have replaced the valves and collets with OE items.
The quality of the AMC collets has also been questioned.
The AMC valve springs seem fine. They are invariably within spec.
Unfortunately the shop that used to do my engine machining has shut down, and I am looking for a shop with VW experience to port and polish my heads and dynamically balance the crank, pulley and flywheel, balance the rods end for end, and match the pistons.
If anyone out there has found a good operator, please let me know - there are a lot of cowboys out there.
I found it was cheaper to buy OE con-rods than to purchase the new stretch bolts and buy and fit new small-end bushes.
You need to do some research on the main oil seal, as there are some bodgy ones out there that fail. I think it was the GoWesty site that has a good discussion on that issue.
Also important to use engine oil for "Classic" engines, as some modern oils are not compatible with the old-style oil seals.
Hope that is of some help, Greg.