Q: I replaced valve cover but have a broken bolt and now am leaking a decent amount of oil

Yes, missing one bolt can cause a bad leak and the only right way to solve the problem is to replace the bolt. Broken bolts can be a mechanic’s worst nightmare. There are several methods that can be applied, but none of them are easily, and sometimes you might start with one plan and have to abandon it for another. depending on how the bolt was broken, you might be able to access a nub and extract it by clamping it with a pair of vice grips, but that only works if you have very good Karma that day. The usual method involves drilling down the center of the broken bolt and driving in a screw extractor. I don’t recommend you trying it unless you have some experience with this sort of thing because I’ve seen many cases of people doing serious damage to the engine castings by getting it wrong. Contact YourMechanic to have a technician come over and look at the problem and recommend a solution.