If you decide to sell it, Advertise the HB on EBAY and you'll be AMAZED how much you get. HBs have a cult following. They were quite good engines. Powerful for the time, and very well made, German craftsmanship.
I must confess to having 2 .15s and a .25 HB myself. I bought the .25 new, and bought the .15s last year for more than they cost when new.

I didn't say I was smart.....