ZX5FS rear hexes/pins question??

[DanB4]

Ok guys I have 2 ZX5's which I bought second hand.

One is a standard ZX5 which has hexes front and rear.
One is a ZX5-FS which I am going to use mainly but it has a pin conversion on the rear, so I can use the wheels from my RC10B4 on the rear.

Anyone know where I can buy a conversion or how to convert the rear of my standard car so I can change rears between my B4 and ZX5's?

Thanks,

Dan