Hopefully someone will be willing to accept a commision for your J15.
In the long term though, conversion of these small 0-6-0s is more a matter of confidence than engineering. A George Watts quartering jig, a set of Gibson wheels and Gibson washers (to reduce the holes in the coupling rods) and you are away.
I would urge you to attend the nearest EMGS area group meeting and let them know in advance that you are hoping for some guidance on RTR conversion – more than likely someone will bring along a sample or three for you to study. Also, it would be very unusual if there wasn’t a demonstation on this topic at the next Scaleforum/ExpoEM.
It really isn’t about engineering, unless you want it to be.