That web-site is blocked because it shows up as 'drugs and criminal skills' according to my ISP!
I didn't have any trouble opening the page.
Has anybody ever done these controller mods before? Where do I start exactly? Do these motors have a board that controls them of is it a hard on, hard off motor?
Lots of people in the forum do motor mods, usually for DC motors. Variable frequency drives are available for AC motors to provide variable speed control, but both the special motors required for variable frequency operation as well as the controls/drivers are expensive. You probably don't want to go there for a simple pumping application.
The pump you cited is submersible and operates directly from the 120 VAC power line. It appears that it is not only submersible, but that submersion is
required to operate the pump. The AC motor is directly coupled to a centrifugal impeller. The outlet size is 1.5" pipe, commensurate with the 1000 GPH pumping capacity. This pump is not capable of variable speed control. It is either on or off.
I have seen similar direct-drive centrifugal pumps used with shallow water-well systems where the lift requirement is modest. Some centrifugal pumps also have inlet pipes for use above ground instead of submersed. Submersible water pumps also usually depend on the water to aid cooling of the motor. This type of pump is useful for recirculating water fountains and, perhaps, very large aquariums requiring 1000 GPH recirculating water flow.
What are you planning to DO with your water pump? What range of flow rates do you need? Why do you want (or need) a variable flow rate? What is the duty cycle of your pump (time on versus time off)? If you can find a pump with a DC motor, then it is fairly easy to control the speed with Pulse Width Modulation (PWM) of the DC supply. We can help with that here in the forum, but I don't think the pump you linked to is suitable for PWM control.