The beauty of battletech is that is can be a game of proxies. If you can take a mech and modify enough to look like another, unless you have a very rules/wysiwyg opponent, it really doesn't matter.
Taking your Rifleman III there, as a side example, i converted the Primative Rifleman to the 1N spec in terms of weapons and overall looks. I've also converted a later model Atlas to look like an Atlas II, and have plenty more examples.
Many builders on this board for example have scratchbuilt the humble Marauder out of countless other mechs, including non-btech sources like CAV.
So yeah, get your mod on and I'm very sure many of us are looking forward to your work!
I have a king fisher on its way in the mail but I was also wondering if I could just modify another model such as:
To represent another config of it