For the mechanically challenged
As John explains above, the combination of a longer rod and adjusting the position of the crank pin will give the necessary stroke to retain the standard bore/cylinder etc. As he explains,the trick[y bit] will be to create a piston profile that creates enough compression while at the same time misses the valves when they are extended - the 690 Rally cam (which should help boost power) has a longer overlap than the stock cam, so things could be a bit tight in there - hence the prototype build and bench test before we go to production with the final specification...
Having wasted my youth on law school and financial management, I would like a little more "schooling" from the folks on the thread-
in order to get to 450 cc. displacement with the same bore, the stroke has to be reduced to 55mm. Moving the crankpin 12.5 mm closer to the crankshaft center achieves the stroke reduction. However, this also drops the the top of the piston below the cylinder deck height at TDC, producing an unacceptable loss of compression.
How does a longer connecting rod fit into the picture? If the rod is lengthened to move the top of the piston to the necessary height (and proper compression), doesn't that also decrease the displacement (piston higher at BDC)?
It's probably obvious to everyone but me - still, a little more education will help.