I did this upgrade and it worked great. Here is a link that describes the process:
http://www.netmotor.de/Bereiche/Zubehoer/Gurte.htm
Sorry it is in German, but the pictures and part numbers still help. I translated it to English by putting the link in Yahoo search. Try this:
http://66.218.71.231/language/translation/translatedPage.php?tt=url&text=http%3a//www.netmotor.de/Bereiche/Zubehoer/Gurte.htm&lp=de_en&.intl=us&fr=FP-tab-web-t
My 81 SC did not have the holes in the deck - so I drilled. Reinforcing the deck mentioned earlier is needed. Large washers will do.
The trickiest parts were:
aligning where to drill the holes because the reels are not separate left and right part numbers. The holes will be positioned asymetrically on the deck. When you get the reels and lay them on the deck you will see what I mean. Just make sure to place both reels on the deck and mark where to drill with a punch before drilling
aligning where to cut the panel for the belt to go through. Patience and a sharp razor knife are needed.
It is a tight fit for an SC panel to go back on with the reels in place. Carrera panels have more height, but you should be OK.
My reels came with plastic clips attached. I did not remove them because they looked like part of the reel. They were transit clips and needed to be removed for the inertia reel to work correctly. I wish I knew this originally!
It was worth it. I can now drive the kids around in safety.