If I understand correctly you are dropping a 1.9 IDI engine into a chassis that used to have a 1.8T engine, and then adding a TDI-specific cluster.
If so, there is no computer associated with the IDI engine...it's RPM signal comes from the W-terminal of the alternator and won't be directly compatible with your cluster... which is expecting an engine RPM sensor from the ECU/crank sensor from your old 1.8 or TDI engine.
A Dakota Digital conversion box is probably your best option, since you'll need to divide the W signal by something between 6-8X (depending on pulley size etc). If this converter works it will also drive the tach of your 1.8T cluster, although you'd have a 6000 RPM tach instead of 5000 etc. And no native glow plug light.
Speaking of which, you also have some wiring ahead of you in terms of the glow plugs, oil pressure warning etc etc.. depending on the version of TDI cluster you track down it may or may not accept analog connections... later models were Can bus so entirely digital.
You could opt for a completely manual glow plug system... if you check the FAQ section there's a couple of threads on adding a relay and pushbutton.