Lethal alright Marc
....I saw a bloke killed on one of those SL350s at Menai back in the early 70s. He jumped a little pile of dirt that we'd all been jumping all arvo but this time he came down front wheel first and face planted over the bars. When he didn't get up we started taking the piss, thinking he was milking it for attention but unfortunately we were a little off...he'd broken his neck and had unfortunately died as soon as he hit the deck.
Having ridden a borrowed SL350 at Nepean dirt track a few years ago, it'd be the last bike I'd like to use for motocross unless it'd undergone some serious improvements in the frame, suspension and weight areas. Great engine though, it'd be a great thing for a flat tracker or for a different pre '75 motocrosser in a motocross oriented frame. You can still get some serious go fast bit for them.