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The formula is very simple.
If 7k is your desired rpm, and your current speed is ~55 mph, and you want to be doing ~65mph at that rpm,
Your ratio change will be:
65/55= 1.182
You can either multiply the ratio with front sprocket tooth count, or divide the rear sprocket tooth count with it.
13x1.18= ~ 15T - 16T up front, or
49/1.18= ~ 41T - 42T rear.
You'll have to look and see if you can physically fit a 15-16T sprocket up front, or if you can find a 41-42T rear to buy.
Alternatively, you can also put a 14T up front, and a ~45T rear; though the easiest swap is the front sprocket.
Swapping out the front sprocket with a larger one, will mostly only affect first gear start from a stop. It does affect all gears but it'll be harder to notice. You still will have plenty of torque in the higher rpm ranges, save for final gear (5th or 6th), where torque is traded for HP; meaning your acceleration will be slower, but your top speed reached will be higher.. a taller final gear ratio, results in an increase in HP, but it only works, if the bike makes the torque necessary to pull at the speed you want to go. Hence you'll lose top end speed with taller gears if the engine isn't powerful enough to pull the gears.
A 150cc engine with carburetor usually reaches about 60-65mph tops.
A 250cc will reach about 75-80mph tops.
A fuel injected one will reach 5-10% faster than the carburetor version.
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