I have a 503 and i worked out a shorty conversion and after that it weighed in at 1510grams fully set up. Fast....like a bullet with the belt drives. Just waiting for the carbon diff covers as sometimes our gravelly tracks are not overly belt friendly.
You can't buy a thing for Tamiya in AUS and the 503 isn't made anymore from what I read. I recently sold the other 503 I had, lucky I have lots of spares and these 503's are pretty robust compared to previous Tamiya's.