Making a separate thread as this may re-interest people who had given up on TG...

You can edit the FLT file for each mission (e.g.)
C:\MSFS\Official\Steam\asobo-lowalt-riffelake\Missions\Asobo\MRK\LowAltitude\lowalt-riffelake\01_LowAlt-RiffeLake.FLT

find the [Sim.0] section and change the Sim= line to your favourite (e.g.)

[Sim.0]
Sim=SSW TF104-G 28+16
;Sim=Boeing F/A 18E Super Hornet Mrk Asobo

I'm using the SSW F-104 but DC's jets should work fine as should the SW Hornet when it's updated, just find the alias you want from your aircraft.cfg file

Thanks m8

Very interesting, might change some of the landing challenges to my preferred types or the upgraded mod versions (e.g. G36 improvement project, Fenix A320)

You can also replace the rewardscreen.jpg file in the images folder with a more appropriate one...
toyHisc2 by Keith Bedford, on Flickr

Ghost also flies the new plane, in fact I've realised Ghost is you, once you've beaten the pre-recorded track 

All good info.  Thanks.  I am enjoying the new training flights.

Ray

You can change the [SIM.0] value to a different plane in ANY bush trip or mission, discovery, etc.

You have to use the right words to indicate the plane you want to use.

You can find the right words by creating and saving any flight from the Welcome map screen:

Switch to the plane you want at the map screen and save the flight.

Open the .flt file of that saved flight and find the right words.

You can use those words after [SIM.0) in any bushtrip, challenge, discovery as long as it has an associated .flt file.

You can change the weather too, or time of day similarly in the .flt file.