I have a Viscount Aerospace Pro that I've had and ridden since I bought it new in 1977. I ride it now when I met up with the kids and grandkids at camping events. The problem is that the rear derailed has worn out and doesn't keep the chain tension. I need to replace the rear derailed but it is a ten speed with 5 rear gears on the frewheel and I can only find a 7 speed derailed. Any one have any ideas