I recently installed a Champion brand spark plug in my 501. It cost me under $2
I was at Autozone, and they claimed to have dirtbike plugs, brake pads, and other stuff. I looked in their catalogs, but couldn't find a listing for Husaberg, BUT I did see a listing for Honda.
My son's CRF uses a similar plug to my 501.
The Champion plug is the same thread, length, and heat range. THE ONLY DIFFERENCE is the exotic metal in the electrode. I decided to try one. I have noticed no difference in performance or starting. No pre detonation problems, although I do run 100 LL aviation gas, so the extra octane could be preventing a problem. I have removed the plug once or twice and inspected it for any internal hidden complications, but have seen no visible signs on the plug itself. I believe this is a viable low cost alternative to the overpriced oem plug.
I'll pull the plug after my next ride for another another inspection. If all looks well, then I will share the plug model wih you. I don't want to post it until I am absolutely sure it won't cause an engine problem.