You're obviously happy with the burb in it's current form. I'd put a new roller cam and lifters in it and call it good. Cheapest option yet. The loss in fuel mileage will definitely offset the cost of a conversion. I'd call cam manufacturers and get a good computer compatible performance cam and drop it in. You'll be done out the door for under $1,000 if you have someone else do it. 139K on the odometer means that engine is finally broken in.