Okay so if you are reading this guide, it is probably because you struggle with the last race in this DLC.
First of all i actually do not think you can achieve a time below 1 Minute and 1 Sek (what you need for gold) with the motorcycles you can choose/buy and only upgrading them to the max of 470pp.
So here is the strategy i used to get a time of 1 Min and 0.895 sek.
First get the 500 V Due bike. Then upgrade everything to max. After upgrading everything go into time attack with the bike and adjust the settings u see fit for your riding style. Save everything. Then strip everything until you go back down to 470pp or lower. Now launch the race. If you apply your preset now the game will actually add all the upgrades to your bike that you had while saving your settup. So now your can race with your maxed out bike while you should only be allowed up until 470pp. It is still a close race but this makes it much easier. This trick works also for every other bike and race.
I hope this guide helped you. :)
For the other races i used the following bikes:
1-2: Multistrada 1200 Pikes Peak
3-4: YB8 Furano
5-8: GSX1100S Katana
9: Supermono
10: 500 V Due