I have had a look at the GA head on A series block problem & I think that it can be done, but not easily.
In my own humble view I believe that an A series camshaft can be retained, although machining it off past the oil pump drive would save same weight. The unused cam bearing bores would then need to be plugged with suitable custom pirces in the unused bearing positions to direct or plug off the oil feeds.
In a later, rear distributor engine, the whole cam can be left in place if an electronic trigger device, or even a distributor ignition is desired.
In a 1200 engine the cam can still be short as the distributor drive & the oil pump drive are the same thing.
A standard type oil pump could then be used which would remove the need to make a drive for the GA pump in the timing cover.
A whole new timing cover could be either fabricated or cast [so easy to say, not so easy to do.] without provision for a water pump which then eliminates one of the major obstacles in the drive area for the new cam.
Another means of circulating coolant will not be too hard to do.
Once the front of the engine is cleared & a means of joining the sump, the block & the head with an oil tight cover is made, then it will be possible to more closely focus on the actual drive to the cams & this may not be as big a problem as might be first thought.
It won't be easy, or cheap, but I think that it is do-able
