Ken's Hoonicorn is rewarded during the Style story arc and Morohoshi's Diablo is rewarded during the Outlaw story arc.
If you have got enough into those stories already on your game save the cars will be purchasable from your garage already for the lovely price of 0.
Now once owned go to any race you're comfortable with and win the event with each car, I looked for any (easy) events. I used 'In The Driver's Seat' which is just up from the garage on the map and takes about 2minutes each race. The two cars are pretty slow and terrible.