Depending on your rep level / overall vehicle level performance / difficulty your game is at, it ranges from pro on up ( Performance Tab, Engine Parts ), around rep lv ~45 is when I started getting U parts ( Always High Heat with game at hard mode ), so maybe goes the same for you or you might get lucky and get U+ before lvl 50 if the games RNG gives you all parts and not the same exact part repeatedly.
After you've unlocked all U parts to purchase, you'll start getting U+ afterwards, it's hella long because like I said, the RNG the game gives is absolutely stupid.
After I have gotten to rep 50 and was still getting the same U parts over and over again for crankshaft and exhaust, it took me a week afterwards to finally unlock all purchased U parts and then I started getting U+ afterwards and even then it was another 2 weeks or so trying to get all of that which sucked balls.
When winning parts it'll show as NEW under those categories for engine parts. I've never ever gotten a drivetrain or a chassis part from doing those high heats. Just engine parts.
More info below~
If you can purchase it, the next step up is when you get those parts from high heat OR getting to a certain rep level.
Pro unlocked > Super. ( Or Rep Unlock )
Super Unlocked > S+ ( Or Rep Unlock )
S+ Unlocked > Elite ( Or Rep Unlock )
Elite > U High Heat ( Easy / Medum / Hard mode Heat Races, Level 3 hard mode grants higher chance at a U Engine Part that boosts performance. )
U > U+ High Heat, Hard mode gives a greater chance to win U+ )
Overall probability to gain U parts while trying to go for U+ on Hard mode I'd feel like is some what moderate.
When night racing start a high rep event on easy ( I-Plane, Axoim, Ring Racer, Spirit of Oz ), every heat race should always be on hard mode for the higher chance for U+ Performance Parts, otherwise, you're going to be farming NOS and Forced Induction kits repeatedly on easy / medium.
Level 3 Hard mode probability Chance
NOS Kit U - Greater Chance
Forced Induction Kit U - High Chance
Engine Parts that boosts Car Overall Performance Level - ( U to U+, low chance )
Level 5 Hard Mode Probability Chance
NOS Kit U - High Chance
Forced Induction Kit U - High Chance
Engine Parts that boosts Car Overall Performance Level - High Chance due to Heat Lv 5. Greater if done on Hard mode.
Through playing campaign repeatedly, this is how I've been getting my parts because I was sitting at Elite from Rep 40~45 and I've been doing high heat since like rep lv 20.
But then again, I'm on PC and it might be the consoles that are somehow wildly messed up.