Clio RS 200 - Gearbox issues and upgrade suggestions

Hi all, I recently purchased a Clio RS 200 Cup. Prior to my purchase the owner had some issues with 3rd and 4th gear sorted As well as a new clutch installed. However, over the weekend 5th and 6th gear decided to make like a French factory worker and stopped working. I am now wondering if I should consider any alternatives to repairing 5Th and 6th. For example, are there any options for replacing the factory gearbox with an after market unit? Also, with the gearbox out I am thinking about a Quaker LSD. Any thoughts of the Quaife units and suggestions as to what other mods/improvements would be useful while the gearbox is out?

Cheers.