Yeah I know I could just bang in another v8 lump but I'm getting 300tdi ,Mazda and diahatsu coming back in searches ,I'm just wanting cheap conversion done and have a s3 to sell to fund it ,if anyone local to aberdeen can help I'm happy to give them the series 3 or the lt95 as payment ,what's a lt95 worth ?
Have you considered what you'll actually use it for and how many miles a year??
As said above, if you want to fit any other engine, then you'll find it cheaper and easier to switch to the more common 5 speed Rover gearbox, most likely an R380 or the older LT77 if not. But you will need to do other things in order to do this. So will make the swap bigger. You might need to do something with the props, mounts, seatbox and transmission tunnel. You'll also need a bell housing and probably a new clutch plate too.
The most obvious alternative engine is a Tdi, either 200 or 300. These are good engines, but noisy and unrefined. And tbh the mpg difference might not be as big as you think.
Most Tdi's will generally average 25-28mpg in mixed use. A good V8 on carbs probably 15-18mpg and one running EFI 18-20mpg.
If you are doing 10,000 miles a year this difference will matter. But if you likely to do only 2000 miles a year or so. Then the fuel difference is bugger all.
e.g.
Lets say you actually do 3000 miles a year in it.
petrol @ 18mpg = £68/month in fuel
diesel @ 26mpg = £48/month in fuel
Is £20 a month really a deal breaker? Not too mention, swapping gearboxes and fitting a Tdi is likely to cost in the region of 3-5x more money than just slapping a V8 in back in it, if it already had a V8 in it.