Does your motor need service? As mentioned most/all of those mentioned are reputable and good enough for scheduled maint.
If you’re looking to add power, I suggest you wait.
Improvement comes first in software (human driving skills) which can only come from seat time. Then hardware, chassis setup and handling comes first. Learning how to get the power you have to the pavement. Teach the driver to communicate, listen, read up on kart setup and experiment with changes.
Once that’s all set, build a legal blueprinted motor, carb and clutch