you can get a rough estimate or maybe even a close match by going to DMA and typing in the prop size then pick a good brushless motor or anything that will turn what your gas engine does.....then just keep adding cells until you get the rpm your glow engine turns with that prop. it will give you static and in flight. www.flydma.com
then go to p calc. oh and I hope you do know a LITTLE somin about cells and electric motors. if so this should be no prob. and fairly accurate.