Wikipedia suggests the Grob uses a Porsche engine as the basis for the engine in the 109, probably based either upon that in the Porsche 914/4 or the one in the later 912E which, in turn, was based upon the engine from the VW Type 4 cars.
http://www.airbornecomposites.co.uk/fly121.pdf
Power came from a 2 litre/80 hp Limbach, based on the Volkswagen ‘Type 4’ engine – the aircooled motor you’d originally have found under the rear luggage compartment of the 411 (remember them?) or more expensive versions
of the VW camper van. This gave the 109 just about enough performance, but Grob always wanted more. When Limbach failed to come up with the goods, Grob home-brewed the 2500 – a big-bore, 2.5 litre stretch of the VW engine that uses heads from Volkswagen partner Porsche’s hotter version of the Type 4 to give 95 horsepower at a reasonably propeller-friendly 3,500 rpm.