Not trying to insult your intelligence at all, but when you did the compression check, did you hold the throttle wide open? You will get lower readings if you dont have it wfo. Also, on a stock cylinder, im fairly certain you dont want to go 68mm. You will have issues. Usually the stock cylinders should not be bored more than 66mm. Im hoping you meant that you were going to have it re sleeved and start at 64mm. 4mm bigger will not make much of a difference in displacement or power. Also, your single carb doesnt need to be much more than 35mm. Anything over that on a stock cyl is a little overkill.