Hi, have been reading this post with great admiration for your work, fantastic work!
Are the little 14/1 gearboxes still available from Bill Bedford? They are listed on his website, but there's no mention of sizes so not sure if it would be too large for rear axle mounting.
http://www.mousa.biz/fourmm/loco/drives/gearboxes4.html
EDIT: Found them on clag as well here:
http://www.clag.org.uk/2nd-gen-mu.html