Logbook - changing details

okay, so a friend gave me his tdi 6n2, it was damaged and the shell was no good.

I recently bought a 6n2 1.0 petrol with engine problems, so I figured I should make 1 from the pair.

I swapped over pretty much everything and now have a working tdi polo.

On the logbook it's still a petrol. The tdi had the lower road tax, was around £40 a year - is this correct?

I do not have the logbook, or the shell as its gone for scrap now.

I have all the engine numbers and transmition numbers etc, and can put it on headed letter to confirm the work carried out.

If I fill in a v62 (is this the right one?) what tax class do I put it in to natch it to how it was on the old car?

My entire intention with this is to recreate the tdi, as its awesome and very cheap to tax. If I have to pay the full petrol tax to me it's pointless

Any advice on this please?