Indeed, the B20E does not use a manifold advance mechanism. It has a vacuum retards function. This is complicated by the fact that the manifold pressure signal is not taken directly from the intake manifold. It is a ported signal taken from the edge of the throttle plate so the pressure signal is not exactly the manifold vacuum signal. However, this is academic since new servos are unavailable. If anybody can get you an exact replacement Eric at High Performance Auto (aka Planetman) in Torrance CA will be the man. Email him directly if he does not respond here.

Note that the B20E engine will run just fine without the vacuum retard function. Plug off the port on the intake manifold to prevent air entry. Set the static ignition timing for 10 deg BTDC and check for maximum advance with a timing light of around 32-35 deg BTDC around 3200 - 3500 RPM. You may need to adjust slightly downward to avoid detonation depending on your fuel octane rating, carbon in the combustion chamber and whether you burn any oil (which depresses the fuel octane).

E models have a vacuum retard. Most recommend disconnecting the hose and plug at distributor and manifold. Was an attempt at emission control, but just negatively affected performance.