Beware of what you are about to do. Not all riser materials are made equal. Some woods are more likely than others to fail, ask Ron or Greg about which riser materials they recomend for bow bolts. I would think a retrofit bow bolt would be tough to do on a shrew handle. You will not have much room for error. I think the smaller size bow-bolt would be required.
Think of this: If your retrofit fails you will be out the cost of the bow bolt and the cost of your Samurai. A used Samuri is worth at least $400-450 if its in decent shape.
If you buy a new shrew with the bow bolt upgrade you are assured of having a great bow for about $300 more than you could do it yourself if you don't buy the bow bolt and sell your Samuri. I have cut a few bows in half doing aftermarket takedowns (connexion hinge and crusader type bolt) and I have ruined a couple of cheap used but otherwise good bows practicing before I got it right. Save yourself the disappointment. The reward from the satisfaction of doing it yourself will not last nearly as long as the despair from desecration/destruction of your Samurai. It is a big gamble against tall odds for small gain in my book.