Can you properly test for overheating on the ground?

I would suggest to learn problems associated with extended ground ops.   See..... https://vansairforce.net/community/showthread.php?t=194385&highlight=vapor+lock 

Do we need airflow over the radiator or is prop airflow sufficient for cooling? 

I'm running 100LL so I don't think fuel vapor is causing any issues. I'm also at sea level OAT is about 45 deg F

Nick if you have an overheating problem, on the ground, at 7C, you will have a worse problem on climb out (max engine power/heat generation).

I am into the test flying of a new Sonex/Rotax 912ULS/Airmaster CS prop - temps (after cowling surgery and addition of cowl flap) all good on the ground, even at Max RPM but not so good in the air. Oil temp main problem - have installed a dedicated air circuit for the oil cooler - only just does the job.

Only long term solution - Will have to redesign cooling system location & cowling to suit.

This is not realy a Rotax problem - its all about air flow & cowl design.

How long did you run it on the ground at full power? 

Were oil temps and CHT both high in the air?

It didn’t overheat prior to a carb rebuild, new muffler and fuel lines and cooling system flush. We’re not sure what could be causing this. 

I think I may be getting the picture/message - this is an unmodified aircraft, that previously had no heating issues,  that has recently started to exhibit overheating - correct?

Overheating, in Rotax 9's, is not usually associated with fuel (carburettors/ULP/AvGas) Unless you have somehow changed the carburettor settings to lean (reduced the fuel to air ratio) causing a hot burn. This would be reflected in high EGTs, probably hard starting, rough running near /at idle, light coloured plug electrode insulation and potential for knocking/preignition at shut down.

High oil temperatures, in a previously satisfactory cooling system, may be the result of reduced air flow over the cooler (contamination of cooling fins, deliberate or inadvertent partial/total blockage of  air flow) reduced oil flow through the cooler (restriction/blockage due to air, collapsed pipe, internal build up of "sludge" )

High coolant temperatures - due to above problems and again due to external air flow/internal restrictions.

Correctly serviced Rotax 9 cooling systems should never need to be flushed. The same goes for oil systems (engine on ULP) BUT if abuse has occurred, or high percentage AvGas used, sludge in oil reticulation systems can occur & cause reduced oil flow.

Correct "purging" of both oil & cooling system must be done after every total drain/refill of liquids, to minimise the chance of an air lock in the system. Air lock will significantly reduce the ability of the coolant/oil cooling system to function correctly.

I am not sure what symptoms a partially blocked muffler might  cause - speculation - hard starting, inability to idle at normal rpm, increased back pressure, poor combustion, low power and possibly heat due to combustion gases being retained for longer than normal.