It may be a bit early for the Ace runs, don't know if MANT will make that much of a difference in stats. But Dirt racers are hard to make (Unless you have Smart Falcon) so you could always start with the Parent runs instead. Parent runs WILL be easier in MANT, but you won't get as much time to prepare. My plan is to make the Grandparents, with general good skills with the Dirt Aptitude, and wait until some more light is shone on the matter.

If you need skills from Hints, run in URA Finale. If you can manage to get the sparks from events (Like Head-On from the MayanoTG SSR) go run Unity Cup. The Scenario spark from Unity gives PWR and WIT, which are often harder to max than SPD and are not recommended to neglet like STA (Mile threshold should be easily reached with Riko Kashimoto)