Bike rental is always an option too! I know Richard de Groot who did it this year too, in Malles Moto. Rented a sorted WR450 from Rallybikecenter, spare engine and wheels.
No need to invest money in the bike for a one off race after which the bike will be shelved, invest money into making it to the start and finish! If you have spare cash from not building your own bike, move from Malles Moto to a team that can do your service and carry spares.