I would go with what I know. The bike has some age on it and no one is clear on past oil types and changes. I wouldn't instantly think the worst however, no evidence of what to expect. I would simply go with flushing oil and seafoam at least twice. Drain the oil in a clean container and inspect. If I'm not flushing anything alarming, do the top-end and call it a day. Of course this is my opinion and what I would do myself based on what you have said.
well if a machine is smoking, which is NOT normal
it says, one of two things
one its got a LOT of miles/hours/hard use and things are worn out
which also means to em ODDS are HIGH, if top end patys are worn out from LOTS of hours and miles
so are many other parts below the top end, as they ALL seen the same amount fo wear and tear
OR the motor was sunk and dirt and crap ATE the top end making it smoke
in which case Flushing the motor is, a GAMBLE
some get lucky , MANY Don't
SO< if your a water riding deep mud guy, then flushing is more likely the better option, run it till it needs bigger repair work, have to pay to play right??
OR if your a guy that wants it to last for many many many yrs, it might be wiser to open it up(if have skills or pay to have done)
and have it done right, and then its good for 20+ yrs of life if you care for it right!
this is the issue with buying used many times, you look at something, it looks nice, story being SOLD is , it was babied all its life, driving by 10 yr old "BLAH BLAH BLAH" just easy life
when it was maybe a all lie's
and then you buy it, drive it a few days or hours and BOOM< its smoking or worse
the lie's then show themselves and you now have to make a call, fix or BAIL
pending your skill level, and or wallet ,m tends to answer this call
as options again are really FIX< or run into ground till dies, OR Sell it and try again
fixing it RIGHT< leaves you in the end with a solid ATV you KNOW is fixed right and should last a LONG time
NO amount of flushing will be 100% successful in removing crap inside a motor
, it just doesn't happen it might remove enough to run a while, and for some a LONG time
but its a GAMBLE