A chap on another forum has just finished building one, as per plans bought from Myfordboy, and utilising the motor and some other parts from a cheap and rattly Chinese drill press. He's delighted with it.
There's an interesting one shown in the forst photo here:
<
http://www.model-engineer.co.uk/sites/7/documents/westbury-hacksaw-1.pdf >
If that link doesn't work (you may need to be a subscriber to see it), It has no vice to hold stock. Instead, the saw bolts on to the moving tenon which sticks out of the back of a largish bench vice, the stock being gripped in the vice itself.
One thing which occurs to me is that to deal with large stock, the stroke of the saw will need to be fairly short. Then, on small stock, only the middle section of the blade wil be doing any cutting. To even out blade wear on small stock, maybe it would be an idea to have a crank pin which can be bolted in at various distances from the axis of the crankshaft.
Andy