NEMA frame sizes are not not phase specific, if you can find a single phase motor with enough HP and same frame it will bolt right up even the shaft size will be the same,,BUT it is unlikely you will find one. A single phase motor of X HP will be quite a bit larger than a 3 phase of same HP, Voltage and RPM. Is it possible to covert the motor, yes, practical? No. VFDs get cheaper by the week it seems and they are a great means to take single phase to 3 and gain variable speeds at the same time.