cactus jack is 100% correct, seals will be fine as they are rated to take petroleum products, provided the engine has not been run it will be fine, as recommended drain the engine, i would remove the oil dipstick whilst doing this to get some airflow through the crankcase , when the drain plug is out, poor some oil down the filler tube until you see clean oil come out, then fill it,
you need to be sure you have got the majority of the old petrol out or it will break down the new oil and you will be looking at a big end failure very quickly.
if you get the thing running you should dump the oil after a few heat cycles and replace.