Connor I could be wrong but I don't think they did one of R6 FMC, I know I looked for one years ago and managed to get R1 FMC, R3 FMC and R4 FMC all in 1/18th and all by UT models, but I could never find R6 FMC.
Maybe KMD could make you one, if you supply the model.
Transkits are available for R3 and R4, the '98 Monte cars, a transkit used to be available for R6 and P9, the RAC cars, I dont know if they can be got now.
How much do you want this model? The transkits can be got, decals can be changed/made but the Tamiya base model is no longer produced and starting to get more expensive. The transkit only does the spoilers so if you want an accurate model the front wings need to be hand done which would take time to do well.
Andy Gilmore does 1 off model models in 1/24 and can creat graphics, I saw a model he did of Shaws Escort WRC and it looked very well, he had talked of doing Meaghers car, maybe worth a shout.
Also Rubywax is starting to do 1/24 conversions, would you tackle this one Ruby?