I hate to break the news to you SMED, but two riders have done the deed.
The first was Eric Geboers, who won the 125 title for Suzuki 1982/83, and the 250 [87] and 500 titles for Honda in 1988 and 1990.
Sefan Everts was the second. He also won the 125 title for Suzuki in '91, the 250s in '95, 96 and 97 [Kawasaki and Honda respectively]. He won the 500 class in 2001 and 2002. And the MX1 class from 2003 to 2006 all on Yamaha.
Everts also holds the record for the most GP titles and races. He also won the MX1, MX2 and MX3 classes on the same day to be the only rider in history to win 3 GPs in a single day.
And Jeff Ward, Mike Kiedrowski and Ricky Carmichael are the only ones to achieve a similar feat in the US. With both Ward and Carmichael also winning the Supercross title. 2 for Ward and 5 for the GOAT.
Back to you…
But sadly nobody ever won on a World MX title on a Montesa