First of all read the guide below to get familiar with car saves.
https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=1521971991

Keep in mind that changing the opponents' cars will also affect the racing events, as some of the cars are used there, and make them unbalanced or impossible to win due to the entry restrictions (unless you use the cheat to enter the event with any car).
Update - Get the following item to modify club and event cars separately:
https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=3000116124

The opponents' cars are located here:
The files are named as the opponent's nickname, mostly, so refer to the opponents list at the end.

One way is to copy the files of the oppenent of your choice to any career and renaming them properly (see guide at the top).

A simplier way is to use these workshop items:
https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=2584483962
or
https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=1158908510 https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=1207774468 Big Thanks and Respect to their creators!

The first one has the feature of getting opponents' cars and the second unlocks this feature.
Subscribe to and install them.
Then follow these steps:

1. load career "Medek-RP-3"
   
2. go to "Browse racing events"
   
3. click the line "Events - Found: 59..." at middle bottom
   
4. then "opponent cloner"
   
5. then "street cars"
   
6. click nickname of the racer you want to modify and remember it (pay attention to upper/lower case)

Modify the car as you like or create a new one, then

1. go back to main menu
   
2. click "save car"
   
3. enter the racer's nickname (which you remembered above)

Repeat for other opponents if needed, then go to

1. make a backup of the "database" subfolder
   
2. copy or move the new files to the "database" subfolder

Here is the list of opponents' files and their ranking at the start of the game.

The steps in this guide were used to create this:
https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=1401993490


Other workshop items used in the screenshots:
https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=1140422180 https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=1149074734


I recommend using this:
https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=1445498636