One thing aswell that has yet to be said about the bike engines is fuel economy! Yes, they only around the 1 litre mark, but they do drink some fuel!! Just as a rough guide, I'm currently getting only around 30mpg out of my GSXR1000 when ridden with a fair bit of enthusiasm shall we say lol! And thats with a bikes weight of around 180kg. And as was previously said, 400kg may take some achieving in a car (although very possible).
For a daily driver, i would be tempted for some type of car engine. For a track toy, bike engines are awesome, but are tiresome on the road.
Just off the top of my head, taking into account the cc limit, hows about one of the old D15 honda engines?? They are dirt cheap now (£100 would get you an entire car!), and no end of people have tweaked them, along with turbo charged versions, but then the fuel economy may suffer!!
Just throwing some different ideas out there
I assume the 1545cc is the absolute limit, and a 1.6 couldn't creep in? even at 1599cc? :p Just opens up an entire different range of engines!
-Gavin