Item description:

The main thing you can do is repeat all of your actions, such as kill one oldie with each of the pickups, this will pop the trophy related to that pickup you've gotten. If you run over the Bicycle Kid then you'll get that trophy too. If you get replay the last race you'll get the "6 Feet Under" trophy for completing the game as the Grim Reaper, which is odd to earn because you won't earn the "One Foot in the Grave" trophy for completing the game in the first place.

Step 1: In order for you get the trophies looking normal you should start a new game & complete it. This will net you the "GrimAced" trophy, for winning your first race. Along with a few other side trophies.
Step 2: Load up downloaded save & go to the Garage, buy something or just mess around in the menu changing stuff. This will get you the four trophies tied to upgrades & the garage, "My Kart is Augmented", "We Can Rebuild Him", "Million Dollar Granddad", & "Sweep".
Step 3: Getting gold in all races, this step was already done. You may need to this yourself although I'm unsure. I couldn't get the "Death Defier" trophy to pop, although the others would pop by replaying the race through story mode.
All the remaining miscellaneous trophies can be earned simply by replaying a race & completing the action once more. Such as the kills, the "Batteries not Included" trophy, & running over bikers/skaters for their trophies. All in all, took me 45 minutes on a new test account to earn platinum for this.