I broke my IB5 gearbox on my Orion Mk1 with a Zetec 1.8. Tomorrow I will pick up a Mk3 XR3i gearbox with clutch and flywheel. As far as I know that's all I need, but I'm not an expert. So is there anything else that I need?

 

You will need the zetec flywheel for the crank position pick up on the back of it, pretty certain the gearbox will be fine with your existing clutch too