Alright here is my opinions on such a bike... but let me tell you a little backstory.
I have a 2004 CRF250X. I have a 06 250R cam in it with R ignition timing and rev limiter +1000rpm (pink wire mod). I ride the piss out of it have 600+ Hours on it, not afraid of hitting the revlimiter here and there, and on an all original top end, you read that right. ALL ORIGINAL TOP END (minus the R cam and exhaust shims... of which my exhaust valves have moved the opposite way of normal... they have loosened so i have had to go only 2 shim sizes bigger over the course of 9 years... typically exhaust valves get tighter and zero out)
Anyways my bike might be magical, my buddy and his boss who run a VERY reputable motorcycle shop just look at me and go wow. BUT i attribute it to a few things.
1. Oil changes, and do it VERY OFTEN. The CRF250R/X and 450R/X have a very small amount of oil that lubricates the motor... 7/10ths of a quart in the 250x to be exact. This small amount of oil gets PUNISHED... 13.5k RPMS and a piston thats got a VERY SMALL skirt which will lead to oil burning. In short the oil gets chewed up quick (both literally, and chemically) and you must keep an eye on it. so for me.... 50 track miles (1 day of track riding), or 120 miles of trail (2 days of trail riding). Transmission oil gets changed after ever 3 to 4 times that the engine side does.... oil filter gets changed every transmission oil change. Running shell Rotella T 15w-40 standard oil. Special fancy synthetic motorcycle oil will not do you any good... you just cant combat the small amount of oil in the first place and high stress inherent in these motors.
2. Dont ride it like a 2 stroke.... stop banging off the revlimiter, hitting it from time to time is fine... but just blatantly smacking off the rev limiter for long long periods of time which i still see today will ruin your motor. All you have to do to fix this, SHIFT THE DAMN BIKE.
OK now that i have gotten that out of the way, let me say what i think about the bike YOU are talking about and how the above relates.
You are looking at an 04 CRF250R.... the first year of a honda 250 4 stroke race bike. ALL of the guys riding these bikes used to be on 2 strokes, which were simple motors that were easy and cheap to rebuild. The problem with that is that same mentality went into riding these new 4 stroke race bikes... so you had guys on em banging on rev limiters like no ones business cus it sounded cool, and guys that never did oil changes letting oil get completely trashed. And this is why alot of the early race 4 strokes got a bad name for failing prematurely... i will say they are great and very reliable i have living proof.
In essence i would be weary of this bike.
Especially when the person says it needs an air filter (a bad air filter will let dust in and this will CHEW VALVES and SEATS up big time)
At the best, you get away with reusing the cylinder, replacing piston / pin - piston rings, replacing head / valves, and MAYBE reusing the cam. ~$500-700
More than likely you are looking at a new cylinder, piston / pin, piston rings, head, valves, springs, cam, cam chain. ~$800-1000
But really... id bet you would be into the above, plus a new crank, new rod, and all bearings associated. I bet the side to side slop of the rod is WAY out of spec. ~$1700-2000
Questions to ask the guy about this bike:
- How many hours on it?
- What kind of riding (track or trail)?
- Why does he think it needs a top end?
- What was his maintenance schedule?
- What has been done on the top end?
- Have valves been shimmed? if so which way were they moving?