Love it, had one back in about 1981 or 82. An SL125 with a big-bore taking it to 140cc (think piston was from a big bore kit from the 500/4). Great bike, did lots of trails, enduros etc on it. As you say, it just kept plugging away all day in the mud. Had a massive 4.00-18 tyre on the back, could balance on it for hours.
Phil