Misfire and codes P0420 and P0171

Just to be clear, you have removed the catalytic converter. If that is the case, you are going to have to ignore the cat efficiency code. If you don't have them then the o2 sensors are not going to see the reduction in emissions so it is going to say it is not efficient.

The fact that you have this could be causing the lean code as well because it is thinking it needs to lean it out to compensate for the rich exhaust.

However, the main reason for the P0171 is a vacuum leak so we should start with this.

Here is a guide that will help with this:

https://www.2carpros.com/articles/repair-lean-mixture-codes-p0171-or-p0174-on-some-manufacturers

https://www.2carpros.com/articles/how-to-use-an-engine-vacuum-gauge

Once we know this info, we will have a better idea what direction to go. So let me know what you find. Thanks