Compare the speed/torque of the engine with the speed/torque curve of your candidate motor.
Using this
http://www.simetric.co.uk/si_cc2hp.htm
I tried to come up with a single formula for all cases but it seems to be impractical. The actual/predicted value in some cases is too far off.
E.g., for a known application (leaf blower),
cc hp ratio cc/hp
27 0.95 28
27 0.95 28
27 0.95 28
27 1.1 25
27 1.1 25
and these points can be fitted to a straight line and a formula for that line can be calculated using, for example,
http://www.shodor.org/chemviz/tools/regressionjava/
so intermediate values of hp or cc can be calculated.
Worst case, you can measure the cylinder displacement of the existing engine.
Since you know your application you almost certainly can come up with a motor that meets or slightly exceeds the engine specs.
Another possibly easier option:
Sawmill blades seem to be within 2' and 5' diameter. Using this search
http://www.google.com/search?client...+diameters+"three+phase"+hp&ie=UTF-8&oe=UTF-8
post at least three blade diameters and hps from commercially available setups, with blade diameters above and below your application size.
This was done, more or less, on a thread where the OP had a 1450w motor to be used as a table saw.