Just did a similar transplant, put an 01800 into my 01600 both were carbed. As long as they get fuel the same way carb or efi then the gas tank will work with both if the 800 is efi (I'm assuming your 500 is carb) you will need an efi tank (fuel pump inside tank). If both are carbed you will need the 800 carbs as they are a different size to fuel the bigger mil. Here is a list of things that you will need to change from what I can remember:
Motor mounts and brackets located in different spots
All electronics for motor
Main Harness (might be the same check #'s from Alpha or Browns)
CDI, Voltage Regulator, APV condensor, APV regulator
Carbs/Fuel Pump
Pipe/Can
Airbox
Either a mag side belly pan from an 8 or 900 or change the hole location in your existing pan as the hole location is different
New gearing
Your servo motor will work fine and your secondary will definately work with some adjustments, your primary may not depending on what the taper is on your 02 crank, I know they did an update on the crank taper for the 02 800's not sure on the 500's so the primary may not fit. It is time consuming but well worth it when you hit the fun flipper IMO. I was able to do mine with minimal cost out of pocket by selling the parts that I had to change out. Hope this helps and good luck
