first of welcome to the site
next, this is info you are NOT going to like
ONE< when you SINK a motor, you ingest water and dirt and crap with the water
so, FLUSHING a motor is NOT how to fix things, its a gamble at best!
next is, if you RAN motor with water/crap in it, you could do damage from the first second it enters the motor and there after!
so, flushing afterwards is already too late many times!
the only TRUE way to fix a sunk motor is to take motor apart/split cases and CLEAN the inside of things
if you do NOT do this, you are gambling and some times you win, some times you loose, and it sounds like your LOST
so, the options here are,
one do a compression test, see where your at there
, or just tear down the motor, clean and inspect all things, as running a motor with crap in it(even after several flushes) can do damage to many parts inside the motor, and could ruin more than JUST the top end and its parts!
again NOT what you were hoping to hear I am sure, but it is what it is!
also, if you want better specific information, you also need to list EXACT year and model ATV this is!, there NOT all the same!
BUT a sunk motor is a sunk motor and the above applies to all of them!
there is NEVER CLEAN water that you drive a atv into, atv stir's up crap, that ends up again getting into motor and eats motor parts!
tear down and rebuild is needed here IMO