I've seen units offered by JC Whitney but I doubt they are anywhere near as nice as the stock Bosch units. The after market system I saw involved using a small magnet on the driveshaft that tripped a senor to regulate a servo attached to the throttle cable.

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LRG
1987 300D Turbo 175K
2006 Toyota Prius, efficent but no soul
1985 300 TDT(130K miles of trouble free motoring)now sold