it is not road legal - without investigating further, it would be classed as a moped, proably need SVA, then MOT, tax and insurance.
I don't really like the chain arrangment either but it is not a bad attempt, not too expensive either.
I have often considered fitting a model aircraft engine as assistance up hill - but never got any further to see if this would be affected by the same lawas as for petrol/diesel powered 2 wheeled vehicles.