Hi Ron,
That is a remarkable piece of work... very well done.
I do share your concerns that the small vee engine is not going to be power enough for it and a larger one may well be needed even if rebuilt built from Ali.
The only issue with a bigger engine is the need for more steam, hence a more powerful boiler will be needed...catch 22 again.
If you do feel like going to a slightly larger engine then I may be able to help... take a look at the 2 attached PDF's
These engines I originally designed to power a couple of Peter Angus' 45mm gauge garden steam locomotives and work at around 40-45psi.
If you fancy having a go at building one then I can search out the complete drawing set for either version and get them to you... I can also provide gun metal castings for the cylinders.
Keep up the great work.
Best regards.
Sandy.