You can find dodgy mechanics randomly at various docks through the universe. These mechanics can offer a number of service, however be warned, their services are not always succeeded.
They will appear as mission targets with the think icon
Overtuned Drive Core
The mechanics can offer to install the Overtuned Drive Core to increase the speed of your ship.
When you accept, your account to be debit if you have the available funds. Then dock at the target station and talk to the mechanic again to install the drive core.
Once installed you can activate or deactivate it at any time.