You must update the details on your registration certificate (V5C) to tell DVLA about important changes you make to your vehicle, including engine changes. So you will need to give the DVLA evidence or written confirmation if you fit that Toyota 2.0-litre to your car. You'll need to provide either a receipt for the replacement engine, written evidence from the manufacturer, an inspection report provided for insurance purposes. Mark any changes on section seven of your V5C, sign it and send it to DVLA (Swansea, SA99 1DZ). If the change isn't listed in section seven, fill in section four and send it DVLA with evidence and a covering letter explaining the change. The DVLA with then contact you to confirm the change or tell you if it needs an inspection of if you have to pay more vehicle tax. I have asked the DVLA exactly what this change means for your situation, but haven't yet had an answer.