Hi there, I have a polaris 2003 scrambler 500. I have gas going in to crankcase and there is pressure built up in crank case. this is causing oil and gas from crankcase to go up through crankcase breather to air filter box. There is a lot of gas mixing through. Could i get an idea what is causing this?

gee is it a
2001 Polaris Scrambler 500 4x4 atv real if yes ask maker for that Polaris.
here
USA site, here, so if not USA change to your secret country

https://atv.polaris.com/en-us/sportsman-xp-1000-s/

yes it can so can anti-freeze but usually gas from carb floats sticking and leaking back into cylinder can cause hard starting and can clean oil off bearings causing damage https://www.polarisatvforums.com/threads/gas-in-oil.10700/

5 years standing is a long time. Carburetor float will be stuck. Pull the plugs and see if they are wet and also if you have a spark.

Strip and clean carb and jets.
Check fuel tap and fuel delivery.
Drain any fuel and replace with new.
Replace the spark plug.
No lights is odd. Check if there are fuses?

Manual

https://www.google.com/search?q=2000+polaris+scrambler+500+service+manual+pdf

Parts and Parts Schematics
https://www.polarispartshouse.com/oemparts/l/pol/50343730f870023468a741cc/2000-scrambler-500-a00bg50aa-parts

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restricted water flow around the engine flush it out with a hose pipe

If a tune up doesn't help, rebuild the carb. the ethanol in gas ruins the gaskets.

drive gears are worn loosening drive chain

if your four wheeler is back firing you may have water in your gas or your gap on the plug

First - Did you find ALL the pieces of the broken piston...if not...STOP and FIND them.. they are in the bottom of the crankcase...you engine WILL NOT live long if you have not gotten ALL the pieces--even the TINIEST ones out!! You mat have more damage than just a broken piston.. something caused it to break.. you may have cracked the crankcases...may have damaged the main bearing(YES) and/or damaged the crank bearing.. see if gas in leaking into the transmission oil.
There is an arrow on the piston that faces toward the EXHAUST port. If there are hole(s) in the piston, they go towards the INTAKE.
Do some more searching and repost the results for review!