hi steve ;
you could try filling it with gunwash thinners [cellulose thinner] for a few days or more and soak it , give it a shake every now and then .
or try nitromors paint stripper in the tank .
after washing it out and if it cleans up reasonably you can fill tank with malt vinegar and leave for a few days , this will clean up a lot of the surface rust if there is any .
you may need to repaint the tank if the thinners or the stripper cause it any problem .
as a young lad i remember riding a wisp with that spinning chromed auto clutch and the bike pedals you had to use to get it moving , in those days all the boys wanted japanese or italian mopeds but nowadays one of those wisps would be great for taking a ride down the lanes round here and looking around .
i upgraded to an NSU quickly with 2 gears and then a suzuki 50 , but the old twist and go was more fun .
would be great to see more of the older mopeds around on the roads these days .
all the best