Given that it was a conversion from a 60ft brake composite anyway, the obvious choice would be the interior from to the appropriate Bachmann birdcage vehicle - bit of an expensive option however. I don't think the interiors were altered in the conversion and you really have to get close up and personal to see interior detail in 4mm especially with compartment stock so you should be able to knock up something acceptable with Plastikard or the like.
Kubes