Helmets and tyres (and lots of other things) have their positions controlled by GP4 Tweaker.
Is the '86 mod a CSM mod?
If so, there might be an option in the CSM configurables menu for 'Real-time editing'.
Turn this on and, in game, use the TAB key to cycle through options, and use U,I,J,K,H,M to move things.
In-game, if you press the '~' key, information will appear on the left of the screen, giving you coordinates for all the wheels, the helmet, and some other things.
Make a note of these once you've made your changes, and then exit the game.
Find the mod's folder, and then you'll have to look for the tweaker files.
These can be in several places, usually
[MOD Folder]/Data/Tweaker/
In there, look for 'onboardcamera.ini', 'helmetpos.ini', 'tyrepos.ini' and, if you need them, 'cptdashpos.ini', 'cptsteerpos.ini', 'steerpos.ini' and 'armspos.ini'
If any of those are missing - usually it might be the 'onboardcamera.ini' - look in the folder for each car; sometimes modders put the camera information with each car:
[MOD Folder]/Data/Cars/[TEAM]/[TEAM].ini
So, for something like the Mclaren, it might be
[MOD Folder]/Data/Cars/McLaren/McLaren.ini
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