Guys, Do yourself a favor, use a good hardened tool steel for your jackshaft like Sjg22 mentioned. We have used the 4140 ground and polished (what we preferred) that you can get from Alro Metals, (a little pricey) on nearly every engine combination from Jawa's to Sudam's with no failures.
Now, regarding the Sprag hub question. We tried it about 10 years ago on a UAS setup and quickly abandoned it, but not for the reasons mentioned. If you use one on a large engine, it will give you the most unpredictable and wild ride you have had in a while and yes, it does affect your braking. Oh, and by the way, the hub will require more than one bearing..... Hope this helps your decision making........