Looking at the various Menzies clan tartan patterns, none of them appear to be a match, based on these HD screenshots:
You can find a ~90% close-enough match in vintage shirt searches, but my eyes have started to bleed looking for a 100% exact match. This is usually the point at which I try to recreate the pattern and print the fabric myself. Thankfully, spoonflower.com allows for custom printing on fleece.
This is my best guess at a tiled pattern, extrapolated from the screencaps:
I still haven't given up on finding a vintage shirt, but once I do -- I always do -- I'll upload the pattern to spoonflower so everyone can order some yardage if they so desire.