Of course, there's always the upgraded hop-up or rail adapter
(here).
And there's also a tight-bore barrel available
(here) - do a search for MK1 and it's the second item on the list. *edit: actually, that's not the tight-bore I was referring to- I seem to have misplaced that link...
I've seen some neat MK1s with wood stocks, longer barrels, etc. So while the gun itself isn't really a gun you can "upgrade" or modify much, it still has some neat options. And it's a very distinctive gun, IMHO. Of course, I'm biased.