A front shock isn't necessarily a problem. The bigger issue is, what are the shocks / forks made out of? Steel or aluminum? You can easily check by using a magnet. If they're aluminum and the magnet doesn't stick, you'll definitely want new forks. If they're steel (in particular, steel "dropouts" where the axle mounts), you should be OK. Lots of people install front hub kits on front shocks if they're made of steel. If you're still worried, purchase a nice torque arm to provide some extra security...

As long as you're going with a low / moderate powered motor of 500w or less, it's very unlikely you'll have an issue with steel forks / dropouts. This would apply to the Wilderness Energy kits, e-BikeKit, Amped and some Crystalyte kits. I'd be more worried if the motor was over 500w (just my personal opinion)...