You could try finding a broad gauge locomotive that is similar to the one you want to convert, and change the bogies in the config.txt file. Remember to clone the asset first so you don't wreck the original.
You didn't specify whether this is a diesel or steam locomotive. This can be difficult with steam locos more than diesels because of how the bogies and running gear attache to the model. A diesle, of course, will be the easiest.
One of the problems you may run into though, is the attachment position. You can use AssetX and the Attachment maker to adjust this position. There is also a way, within the config.txt file, to adjust the height of the bogie. Just remember that if you pull the height up or down too far, this may affect how it looks when used with the loco. A set of bogies pulled down too far will be running in the ground, and those that are pushed up too high will be running into the under carriage of the locomotive.
Your rather vague title may have kept people from answering your question immediately.
John