The question is: Digital in the sense that it displays in a non-analog format, or digital in the sense that it uses digital signals from the motor to convert and display the information?
I suspect that, if both the motors are pre-2006 (I think that's when Yam came out with their Command Link scheme), it is an analog guage that displays digitally.
Assuming you can look at them, you can compare the Yam guages here
I think you're looking at the multi-function guages. The Command Link (which, I believe, won't work for you) are listed separately in a different heading. The site lists the same part number multi-function tach for both the F115 (4-stroke) and your 90-2-stroke, so the answer to your question is yes, they are compatible. As I recall, there's a DIP switch that sets between 2-stroke and 4.