Hello,
To maximize your car's potential, I'd recommend trying to repair your engine before every race - repairing this often will require you to purchase some engines with tokens though (you can also get some extra engines by claiming the daily reward).
Parts are less important and it is noted by some high-level managers that you can fail to repair your car for 1-2 races and not see any significant negative outcome for your car's pace.
However, if you let your car's part heath drop below 8%, you run the risk of suffering from a "suspension failure" and retiring from the race.
You can get more parts, either by purchasing them with tokens, through daily rewards or from your manufacturing headquarters.
Some of the information from my reply has been taken from the unofficial GP Manager guide - https://igpmanager.com/forum-thread/45862
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