Presently, I ride a mild mannered Honda Transalp and a Ural sidecar.

However, I am tired of compromising in this way or that with my bikes and lack the funds to buy a new one. I want a sturdy, reliable bike that can go off road, albeit without needing to win titles...

It should be potent enough to cruise on motorways.  I want torque and I want fuel economy and I don't want Urals 1500 mile service intervals.

So I thought about building my own and considered the following:  
Putting a small size car diesel in a bike frame (eg an old soviet bike frame as they have bags of space for the engine and I can get a free K750 frame if I want), tuning it to suite a lighter vehicle and hopefully improving consumption further still.

Candidates so far are the likes of the Daihatsu Charade 1l triple.

Unfortunately, my imagination far outstrips my mechanical aptitude.

So, how feasible is this?
Is that lump, or others like it, small enough to fit in a bike chassis?
Would the engine be torquey enough to cope with a single speed gearbox?
Could a low ratio gear be put in for difficult terrain?
Or could I mate it up with a Ural gearbox (4 , 1 reverse)?

An idea of frame size can be seen here with a Ural that has similar dimensions, albeit with a standard Ural engine in situ:
http://hammarhead.com/motorcycles/solo-x/