Whatever you use do not go for a 'cruise control' style. You want that throttle to drop to idle when you let go of it. Never, never use a control that holds the throttle at a set position. That's an accident waiting to happen.
Replacement twist grip throttles are available from almost every vendor supplying bicycle engine kits and parts. Check around.
Age and Treachery Will Always Triumph
Over Youth and Skill & "Charlie Don't Ride"