According to FastLaneGaming, there's a P1 GTR coming! (it is NOT the owen's edition because if you look at the back it has that big carbon piece like the real one) :
McLaren | Forza Wiki | Fandom
Top 10 Rarest Cars in Forza History - KeenGamer
Logged in and found The mclaren owens edition : r/forza
FH4 - Everything about McLaren P1 "Owen's Edition" - YouTube
Need For Speed Most Wanted: Car Showroom - LRF Modding's McLaren P1 Owen's Edition | NFSAddons
Forza Horizon 4 | McLaren P1 “Owens” Edition | Full Customization - YouTube
There's a hidden new car in todays update: P1 'Owen's Edition'. Anybody has an idea what this is? : r/forza
Recent Update for Forza Horizon 4 Vehicles - ForzaTune