Get a new fan. The lower the sone rating the more expensive but 1 sone is best and 2 is ok and 3 will be better than what you got. Don't go higher than 3 but a 1 sone is nice if you can afford them. For an 8' ceiling, take the sq. footage of the room times 1.1 to get minimum size for cfm to get your 8 air changes per hour. I would get the biggest you can afford. Remember you want the fan to get rid of moisture as soon as possible to avoid mold and mildew and moisture damaging your house which is why they wire them together with the light to start with which is more important than just getting rid of the poop smell.
Fans need cleaning every 6 mos to a year cause they fill up with dust which gets in the bearings/sleeves and drys them up which then makes them noisy from the lack of lubrication. The vent will plug up and they won't hardly draw air anymore. After cleaning try a little 3 in 1 oil on the bearings. Yours may already have bad bearings but it is probably the old standard cheap noisy ones put in a lot of houses. Remember when fans get noisy, the bearings can be getting hot and be a definite fire hazard especially with all the dust collected around the motor. the covers pull down and are held with spring loaded wires that can be squeezed together for removal.
Also for easier motor replacement, fan motors usually have a plug and receptacle inside of it. You can pull the cover and unplug the fan if you want to stop using it for a while.
Dave has you covered on the wiring. You could accidently get lucky and have separate wire runs because of the fan and light location distance but probably not. You'll probably have to run new wiring. Does your hot/line wires from the main panel go to the switch or to the light and fan? Broan makes a single, double, and even a triple lever switch (for fan/light/heater combos) that fit a single gang box. If you want one with a 1320w or 1500 watt heater you will also need an additional circuit (breaker and wire) run from the main panel. Try this website www.broan.com They also make the nutone line of fans.