I use the O.S. A5 plug myself and think that it works better than the R5. You should seal the engine up with high temp silicone (should be able to find a how to on the engine forums) and reset the needles back to stock specs and then re-tune.
15% nitro is still kind of low CEN recomends 20% with 18% oil, I recomend O'donnels 20% RTR fuel if it is available in your area.
As far as spare parts this list should keep you going for awhile,
GX12 Differential cups
GX15 axle shaft
G84307-01 47T Spur (1st gear)
G84307-02 44T Spur (2nd gear)
Glow plugs
FF063 Screw set
FF012 teflon clutch with spring
FF009 Body mount set
FF046 Center dogbone
FFS003 Unversal swing shaft
FF003 Suspension arms
If you don't have one a temp gauge.

rolland