The speeds you are after can easily be attained with both brushed and brushless power systems, but if you are <2 knots under your target speed, and it is noticeably slower than it used to be, it seems likely that you can achieve your target by getting your existing stuff running as it should, without spending anything.
My first step would be to try to get back the lost speed that it used to have, take out the shaft clean it, oil the bearings and check it spins freely, if not the bearings may need replacing. Clean the gears and oil the bearings in the gearbox, lightly grease the gears, spray the motor down with brake/carb cleaner and oil the bearings. Then test for speed again.
If speed has not recovered with that done, it is likely the ESC calibration with the radio that lost the speed, you say it is an "receiver/ESC' unit" which implies a possibly toy grade all in one unit which I am not familiar with does it have any markings that you could search online to find a manual (or post photos here)? Most stand alone ESCs have a setting to calibrate them to the radio. Does the radio have end point adjustments or a throttle trim? Adjusting those can help if the ESC is not seeing full throttle when you apply it. Some ESCs even have a handy LED that comes on at full throttle so you can check the calibration.
Paul Upton-Taylor, Greased Weasel Racing.
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