You can replace the rear struts with shocks.
You can also install a jackshaft with sprockets, just flip the rear wheel so sprocket is on the right hand side of bike.
You will need to spend some time aligning the sprockets on the jackshaft to match the drive clutch and then the rear wheel...
It would not have a two speed as I now see it has the 60 tooth sprocket. It would be a XL-500 or XL-350.
They use a jackshaft with 2 sprockets on it.
Here is a link to the 1968 Rupp parts catalog: