I have an iPhone 7 that purchased around September 2017. I broke the screen and took it to iFix where they replaced the screen and put a screen guard. Since that time my phone overheats in less than half an hour and when the pgone is hot the screen

High temperatures are not welcome on the iPhone:--> Keeping iPhone, iPad, and iPod touch within acceptable operating temperatures - Apple Support

If you're using your device within acceptable temperatures and it still becomes hot, you should have it inspected where you had your screen replaced.

Regards

Giulio

It‘s probably because of the third-party replacement. iFixit (and others) have no access to genuine Apple parts. The screen you got as a replacement is low quality and is now failing or malfunctioning.

Always visit an Apple Store / Genius Bar to get a repair or replacement done. Now that you went somewhere else to fix your phone your warranty is gone. Apple will no longer touch your phone. Also the warranty is only valid for 1 year, so it would have expired by now anyways since you got the phone in 09/2017. Unless you had the AppleCare+ program.