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anything will go into anything as some poeple have proved, just depends on your level of skill, how deep your pockets are, how much sense you have really.. but the answer to your question in short is yes! It has been done before and it is realistic but not totally straight forward if i remeber correctly. Pretty sure i have seen a build thread somewhere on the net..
seen it done to an Arosa. a G40 lump is a far better engine that any of the std lupo engines, what a hassel i have had with my gf's one.
I did it in my Arosa
It can be done very easely. only one mounting brecket has to be modified (left right [hen you open the hood)). gearbox nd right one are the same as G40 hs nd that's it. everything else will fit. just don't do same error as I did - lupo nd Arosa hv shorter gears than G40 nd if you place his gearbox on G40 loom you will hv car for hill racinag but for highways....auch.
second thing what you can do (if you want to hv car as I described) - you can hv first 4 gears short nd 5th can be longer - just put 5th gear fm some diesel engine. e.g. Ibiza 1.9TDI
you can check here
http://www.polog40.co.uk/forum/index.php/topic,151.0.html
i was gunna use my g40 engine gear box and loom
you wan't hv any problem