Were the Marble Blast Ultra stages recreated in the Marble Blast Gold engine?

So it seems since the majority of models are directly from Marble Blast Gold. Are the physics properties directly from Gold, reverse-engineered from Ultra or somehow ported? Was it possible to rebuilt the levels or were some adjustments necessary due to the engine or differences in physics?

MBG and MBU use the same file formats for levels. At worst, MBU files in a newer version of the level geometry format would need to be tweaked into an older version for MBG, but I don't know if that's the case. Otherwise, I believe it's as easy as copy-pasting.
As far as I know, PQ has the same physics engine as MBG, considering PQ is a modified version of MBG. The physics engine lies in the application file, making it very hard to reverse-engineer. Perhaps the Platinum Team managed to hack in a few tweaks, but I'd think those would not affect existing features of the game.

Weather is correct, PQ is just a heavily modified version of MBG with minimal if any changes to the physics code.
To add to this, Alex Swanson (level designer for MBG/MBU/Marble It Up!) gave us the files for MBU's levels - they weren't taken/reverse engineered from MBU or the MBU PC port. All MBU levels appear in PQ exactly how they did in MBU, differences in texturing/shading aside.

Great to hear that to you got the MBU level files from the level designer.

Sorry, but I don't know how the Ultra physics differ from Gold, if were modified at all? Being a Xbox 360 exclusive and having been delisted in 2011, the game is fairly difficult to access.

I compared the accessible physics settings in the two games (with MBU being a PC port) and found that they are the same. According to settings in MBU, the MBU marble is 1.5 times larger.

Weather wrote: I compared the accessible physics settings in the two games (with MBU being a PC port)

You mean this unofficial port ?

Yes.