FR-S, VA WRX, and BR-Z fans rejoice, ASF Machine makes a rear main seal installer tool for the FA20.
ASF Machine also makes a rear main seal tool for the Porsche 9A1 engine.
A nice tool that can really make Subaru engine assembly a lot easier is this ASF Machine crank holder. It bolts to the end of the crank and the flat square end clamps in your vice and holds the crank securely. This is easier than making a fixture to hold your crank while tightening the rod bolts or rassling around with your crank on the bench while trying to measure torque or bolt stretch.
We use ASF Machines tools in our shop for Subaru engine assembly and you should too! It helps us do a better job faster.