[Sorry I've posted this to "general", as the "modification" forum is bouncing my emails]
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(now moved to the modifications section,,, email for this section is modifications@... not modification@ (webgimp Paul )
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Whatever engine you choose you will also need to fit the matching gearbox and propshaft, and to make use of the extra power you'll
need the axle from an OHC Viva or Magnum.
The fact that the Viva is rear wheel drive places some limitations on your choice of engines. Modern vauxhalls are mainly FWD apart
from the Omega. The 2.0 litre Omega engine would therefore 'appear' to be the most obvious choice if you're limiting your choice to
"modern vauxhall" engines, however it will be a lot of work due to the electronics.
Going a bit older (and hence simpler), there's the Manta/Carlton 1.8 and 2 litre engines worth considering.
Straying from Vauxhall to Ford for a moment, most Sierra engines could be made to fit, 2.0 Pinto, 1.8 CVH, even the 2.8 from the
early RWD XR4!
You could also consider the old Ford x-flow as these fit more easily, parts are plentiful despite the age and reasonable power
figures are attainable. A 1600 X-flow can develop 110BHP with just a cam change and a suitable exhaust. Not huge power, but adequate
in modern traffic and easier/cheaper to maintain than any of the Vauxhall OHCs.
Sid