Mechanical speed controller

Hi all I'm new to this site, and i'm very impressed with the knowledge you guys have.
I'm building a R/C model Brede class RNLI lifeboat mainly from scrap materials, and i'm using some old ACOMS R/C gear, the speed controller is mechanical, it works but there is a crack in the oval shaped part on top that is connected to the servo which moves it forward/backward with 3 stages of speed.
I know there are some controllers that can be fitted directly on top of a servo to control direction and speed so i was wondering if any of you guys have a diagram of how one works?.
Can anyone help please?.

You can buy ready made speed controllers (ESC's) from any model shop - you're not going to build one any where near as cheap or as small! - it's NOT really a trivial task. You also won't need the servo, the ESC takes the R/C servo output directly.