It looks like you can simply buy a new adjuster screw. You don't need to buy a whole cap...
I talked to Les @ LT-Racing a bit ago and he gave me marzocchi part number 850652 for this screw.
Also, you can find 45mm marzocchi fork parts through your husky/ktm dealer... The husky used the 45mm zoke for several years - up to 2009 or so...
It should be easier for you to round these up in europe versus here in the U.S....
jeff