I run both of those cars, race the truck every week, race the buggy 2-4 times a month, never had a problem sourcing parts, Rob at Southwest RC is ya man
http://www.southwestrc.co.uk/
give him a call if he doesn't have it in stock he can get it quickly.
They stock RW spurs which are better than kit stuff, plus you want a smaller spur really to move your motor closer to the gearbox I think I run a 78t.