Loaning private money notes in a Solo 401k

[Olivia Dansereau]

If my solo 401k lends private/hard money to an investor, am I able to charge a percentage of the profits as well as interest? In my state the interest rate is capped at 12%. Also, what about up front fees? I can't seem to find regulation governing those amounts. 

I want to stash as much as possible into my 401k but don't want to get hit with UBIT taxes. 

I also have the option of buying a share of the business directly but was told this would trigger additional taxes whereas lending on a note would not. The investor and I have agreed to a 60/40 split  (I'm at 60%) so I'm trying to find the best way to structure this without triggering UBIT.