I initially installed from l-29upx.zip (FSX Aero L-29 Delfin Update, 4/23/2019).
When I had trouble with that I installed the earlier version, from l29upd.zip (FSX Aero L-29 Delfin, 12/4/2010). That would not load correctly either.
Interesting to note that the developer(s) use more than 30 individual gauge files for all the individual instruments - have never seen that approach in any other FSX aircraft that I have downloaded.
I downloaded both of these versions from flightsim.com. Am running under Windows 10.