OK, so I installed the WAD I found into my PAL Wii, and it runs at 50Hz.
For the record, since it runs at 50Hz, the game runs about 16.7% slower (sound pitch lowered, Mario/Jumpman moves slower).
Normally, in cases where the game actually runs at 60Hz in a PAL VC release, a message appears at the top right corner (click to enlarge):
For the record, Donkey Kong Original Edition was first bundled with a special version of the Wii (a red-coloured one). Later, it was released on the 3DS VC for those who bought a specific game and registered it before a specific deadline (now it's available for €4.99 in the EU eShop) and presumably WiiU VC. Whether it's the same ROM as in the initial PAL Wii release, I have no idea.
EDIT: I just bought it from the 3DS eShop and it runs at 60 FPS there. Seems to be the same ROM as well, having the same copyright date (1983-2010) as the one normally found in a special edition PAL red Wii.