Just some ideas to present for general perusal...
I've been thinking about repowering/upgrading/whatever options for my Trix steamers. For the K4s I used a link I keep for Kato Japan
http://www.e-katomodels2.com/ I looked for the drive for the C55 so I don't have to buy an entire locomotive. While the C55 is not available, the C57 is. The drivers are totally wrong but it does give me an idea of what a C55 drive might cost...5,250 yen or about $54 US at leasst half the cost of the entire loco.
For $41 I can get a D51 drive for a H class 2-8-0. Throw in another $3 for the lead truck. Haven't checked driver size, but performance is a higher priority. No doubt the cylinders are wrong but perhaps they could be swapped for Mikado parts (albeit very carefully).
As near as I can tell there are no Japanese 2-10-0's to improve the I1. Maybe a modified 2-8-0 could be used.
I also have a link for Micro Ace
http://www.microace-arii.co.jp/ Did see a C59 and a C60 (once again wrong drivers) but cannot find the parts department, so I have no idea what the costs are. I cannot read Japanese and apparently neither can Google.
Finally, there is Tomix...
http://www.tomytec.co.jp/index.htm Still browsing this one, haven't found the steam or parts sections yet. I did, however, find the cute little anime figurines. Nothing like a few of them to dress up the train room.