The 12g295 head nowadays is more of an expensive antique, rather like Wolseley front brakes. Unless you are convinced it hasn't cracked across the valves, and you are happy that ploughing huge amounts off the head won't compromise the deck thickness and cause head gasket/warping problems(Vizard is quite clear about that detail!) then go for the 940 option.
If possible, hunt out Declan Burns thread on this from around 2 years back. He covered the whole job pretty well ,with photos!
I am still unsure about the modified 1098 versus 1275 question. While logic says that the 1275 should
be superior, (and I have one waiting to drop in when the weather brightens), the one attraction of the 1098 is its very long stroke, which makes it handy for pulling a moggy traveller, especially if you tend to load it up a bit....
Working, as I have, on the principal that generally using what you already have is the cheapest option, there is a big satisfaction in "just" dumping a 940 head and decent carb/manifold on your engine and noticing the change
. The weak link will unfortunately always be the main bearings, unless you can happen upon a 1098 Sprite engine which will have the 2" mains(rolls on floor laughing)! I know where there is one, but even an optomistic fool like me won't touch it as it shows many signs of abuse (yep, the head is cracked on it!)
Bear in mind that if you later decide to drop a complete 1275 in, (which isn't at all a straight swap when you get to the flywheel end unless it is the spridget version
), you will still have a 940 head that you can sell on, most likely at no loss!