Oil Still Too Cool Even With Cover

I believe I have a thermostat.  See the photo.

Despite that, I still have cold oil...

I would test it then. With it out of the system using a hot and cold liquid you should be able to see if the thermostat is moving/working. It may be stuck and or needing replacement. Maybe the manufacture has a test procedure. If it was working properly you won't need to use tape.

Verify the oil temperature gauge is accurate/reading correctly.

Is there an "easy" way to do that?  Taking off the thermostat is kind of a last resort.

First, let's establish the temperatures you are indicating and compare them with the Rotax operator's manual. People often cite "Yellow", "Green", and "Red" without connecting them to the actual numbers. 

https://www.rotax-owner.com/pdf/OM_912_Series_ED4_R0.pdf

The Rotax operators manual provides the following temp limits for Oil. 

Max. 130 °C (266 °F)

Normal operating temperature: approx 90 to 110 °C (190-230 °F)

Min. 50 °C (120 °F)

Warming up period Step Procedure:

1 Start warming up period at approx. 2000 rpm for approx. 2 minutes.

2 Continue at 2500 rpm, duration depending on ambient temperature until the oil temperature reaches 50 °C (120 ° F).

3 Check temperatures and pressure

3.7) Cruising

Oil temperature

Avoid operation below normal operation oil temperature (90 to 110 °C / 194 to 230 °F), as possible formation of condensation water in the lubrication system badly influences the oil quality. To evaporate possibly accumulated condensation water, at least once a day 100 °C (212 °F) oil temperature must be reached