Check that the gearing is OK for the diesel. I would think the 914 trans is set up for higher RPMs and diesels operate at low RPMs. It would be quite a bit of fabrication to fit the TDI trans to the frame. Diesel will work with the 914 trans, just maybe not optimal on the highway.
http://www.914world.com/specs/regearing_901.php
The spread sheet shows 95 mph at 4291 rpm. The VW TDI makes maximum torque at 1900 RPM so cruse at should be around 1800 RPM, so this would be 1800/4291*95 = 40 mph. This is with 23 inch OD tires.
The Gulf TDI is at 1800 RPM at 65 MPH.