By the sounds of it, its the one off EBay.
All the problems you are talking about are the biggest problems with the Corsa C LET's all are run of the Old ECU (in your case the 1.2)
BUT you can get everything working, the only thing i have come across is the temp on the dash can be a real problem.
If i remember right some of the things you have said are down to the sensor on the standard engine with runs off the bottom pulley, SO if whoever has done the engine hasent put that sensor on this engine your going to have problems.
IF i was in this situation i would just buy a Stack dash with after market sensors :thumbs:

you buy this on ebay?

I have a Corsa C with a 2.0 turbo calibra engine conversion. The power steering, rev counter, speedo and temp guage dont work. there is also the battery light on my dash. do all these things run from the standard ecu from the 1.2 engine? has anybody else had these problems with engine conversions in a corsa c.
any help or advise in the right direction would be greatly appreciated.
thanks.