Used cost on a Stock Moto engine package seems to be around, $2800 however they seem to have dropped a little of late. Here’s what I posted in another thread…
Used about $2500 with a ’99 cylinder, probably $2800-$3500 (or more) with an ’01.
New kit (you assemble and prepare) $2800 for the 99kit, add another $400ish of you want ’01.
Brand spanking new and assembled $3800-$5500 (and beyond).
Rebuild interval is a contentious subject, but I would say 25hrs on a top end and perhaps two tops before changing the bottom would be a good rule of thumb. Many claim to run their top ends much as 50hrs in a stock moto when being used as a practice donkey, tread carefully there. Like I say, opinions as well as situations vary a lot. If you are running hard and competitively look at changing top end every 5hrs or so.
I think most would agree that running a quality leaded racing gas and racing oil will help ensure the motor stays in good shape, especially those bearings that you cant inspect visually down the bottom. Periodic inspections of the internals of any motor that’s having the living daylights revved out of it is a good idea.
Provided your cylinder is still in good shape you could do a top end yourself and have change out of $150.
Chassis wise, if the Birel or Corsa range is working for you with your existing fleet and you have good contacts and support, I think stick with those brands.