Personally, I think the cam profile would be the easiest thing to do. With extended lobes, I don't think would be a big problem in the cam housing, most have a lot of room because of the cam gear. And like Phil said, just regrind the lobes, don't weld them to get the right duration, lift or timing. The difference can be made up on the tappet adjuster. And heavy duty valve springs wouldn't be a big problem either, there very easy to change.
All this is done on larger engines, so why not our smaller ones? I think there are a lot of people out there that would spring for a "hotter" cam. It happens every day for car or motorcycle re-builders, why not us? Think about how many 4 stroke engines are sold everyday in this country. Just the most popular brands, like OS, YS, etc... I wish I could do this.. I think there is a market for this, even though it maybe small, you never know until you try.
Suptter