at 1600 for the quad you cant do any good if you cant do it yourself. If doing a bottom end you almost always have to do a top end with it, plus whatever else may be broke once you got in there. I wouldn't start a motor rebuild myself without at LEAST $800 available. Probably be double that for a big shop to do it, Unless you get lucky and find a guy that can work on them that wont rape you like a big shop would. As for the aftermarkets everything I see is cheap stuff so don't let him suck ya in with the word aftermarket lol
2009 CRF450 Full TI yosh and Remapped
2001 CR250 Single track bike