Here is an update on the progress I have made.
Firstly I would like to thank
rallymaster/Phil for the photos he sent me. Much appreciated.
Secondly,
MPWR, the M1 was used as a procar in 1980 and also there are decals out there for a few LeMans cars that ran in the '80s as well. So you could have done so. The next comp looks to be booze and ciggy advertising and Lauda ran a Marlboro sponsored car so there is an idea for you. Check out Mediamix as Wong has some and I am sure that other online places have them as well.
Now to the progress. It seems slow however the correction of the body has been delicate and time consuming.
As Phil mentioned, the front spoiler/airdam needed reshaping. This car actually had quite a different shape to the race cars and even the next years version of the Motul car. First two photos show the areas that need removing. Right side of opening is the original kit part with the left after modifying. The lip on the bottom was also removed as the rally car spoiler is shown in the 3rd photo. The lip in the middle needs building up and although I have yet to take a photo of it in it's finished state, I have done this.
Next was the modification of the rear bumper. A second air extraction hole is required on the opposite side to the kit one. Also I needed to remove a section in the middle. Rear shot of the car shows what was needed. The black area immediately under the rally plate is black tape holding the plate in place.
The kit wing supports needed both filling of the sides to get rid of the moulded edge and to round off the curve. The image shows the unmodified support on the left with the modified one on the right. I have also included a shot of a real support.
The air vent in the centre of the bonnet looked very agricultural so I thinned out the blades. Again, the left side shows the modification with the right showing the original look. Looking back at this it would have been easier and look better just to remove and replace with pieces of sheet styrene.
The wheel arches were modified with the rear bumper being glued in position before modifying. Filler was used to also blend the upper and lower arch as the kit does not match. Unfortunately I did not take any photos of that work.
After all that the car was primed then painted with Tamiya Pure White TS 26 from the can. Then masked and the interior shot with Tamiya Flat Black with the door frames and window rubber hand painted flat black. All photos I have found of the race/rally M1s show black interiors and underbody.
The decal for the bonnet was all one piece as the next photo shows. First I cut out the area for the bonnet vent. I then cut the individual stripes out. Although this seems silly, the stripes on the decal would not line up on the air vent as per the photos of the real car. I also don't like big expanses of clear decal. Just a point for anyone doing this, the Rennaisance decals for this are fairly thick yet with use of a decal softener they will eventually conform. Just don't use too much softener as they then will start to melt.

And this is how it looks at the moment.
Hopefully I will have some more done over the weekend.
Mal