I thought Thumpers were single cylinder bikes.
I had a BMW F650 GS ('01). I think they were the same as a Fundoro except the Fundoro was belt drive vs chain for the F650. The F650 GS was the ABS version of the F650. They were pretty big and heavy for the dirt bike part of a dual sport. They did OK on the road with the FI models pulling ~60 mpg when ridden less than 65-70 mph. After riding the Valk, the F650 was like riding a bicycle.
The early models back in the 1990's were single cylinder 652cc. Rotax, chain drive.