It seems that WinMount tool from which MOU compressed files (.mou) originate also supports other disk image formats, including ISZ format from UltraISO. So the obvious solution for this conversion would be to use the default software. We haven't found any information about other programs capable of this conversion, so this probably remains your only viable choice.
Convert MOU compressed format to UltraISO compressed disk image.
We found one software available in the database for this conversion.
Microsoft Windows software - convert mou to isz on Windows
Program used to handle compressed archives and disk images
Convert from mou file format
The table with program actions contains information about what each program is capable of doing with their files. This may be a good pointer for further searches for certain file conversions and help you find the converter you are looking for if our search result is insufficient for you. However, it is far from perfect and may sometimes show results which are not really usable because of the nature of the software handles the files. So a conversion that appears as possible, just because of matched actions, may in fact not be possible.