Sorry but the engineer in me cringes when plastic gears / pinions are discussed. Yes they are OK in certain uses but as has already been said the magnets are very strong on an Alton. The plastic gear may last a year two, but if it gives then there will be plastic shards all inside the engine.
Maybe i'm been a bit cautions and loads of owners will come back saying they have run platic gears on Alton's for years OK, but if it was my bike I would be looking at fitting a steel gear.