What is wrong with my car?

I have a 1991 Honda civic, I just topped off the oil the battery and alternator are new.
My car recently started to make a bad spudder sound and it doesn't have the power it used to. Sometimes when I put it in first and take off it lags and takes a while for it to get speed. It feels like it's hesitating, it will give me power then feel like it's gonna stall then give power again.
I have also noticed a bad smell coming from my heater when I use it which is new, and when I brake my heater will blow slower and my lights will dim. That's about as in detail I can get please help!

This car has a computer I believe. The check engine light should be on. If so, have the code's stored there read by Autozone or equivalent. The interpretation of the code would give you a clue if there's a code.

Gee, how many miles does it have? It's only 23 years old, what do you expect. There could be a few vacuum leaks, bad EGR valve, and a ton of other problems.

When was the last time you had the spark plugs AND spark plug wires replaced?

"when I brake my heater will blow slower and my lights will dim"

I wasn't going to chime in on this because I'm not a honda guy but that caught my eye.
You also state that you recently had the battery and alternator replaced. Odd that they would be replaced at the same time. Your comment about the brakes caught my eye because per your description there's an electrical issue. Maybe not the root of all your issues but certainly an issue.

I would do a voltage check on your charging system, including charge voltage, resistance checks on the bat pos to alternator, bat neg to ground, and ground to alternator case. I suspect there's a bad connection in there that resulted in you replacing your battery, Alternator, and maybe even a run-ability issue.