The RPM's are pretty high on the Mamod oscillators, so for what it is worth I find a 20:1 reduction is about right for a realistic speed on the TE's, you are working with smaller diameter driving wheels so that will have to be taken into consideration.
I had overlooked this and it is now an area of concern but well pleased you mention it.
I will try it now but suspect you will be proved right.
If so, as I want to complete my 4 inch before I pop my clogs, I shall have to find it a new home.