Here's my 1977 F2 that I bought as a 450 dollar eBay wreck. It had been smashed, crashed, and set on fire at different times in it's unfortunate life before I bought it, and the only items that I could salvage from the original bike were the frame, wheels, front fork lowers, and engine cases.
With help from some members here, and eBay, and David Silver Spares (and a couple of swap meets) I was able to return it to it's former glory. (with a few sensible mods to aid it's longevity) The F2/F3 was a good bike if well maintained, (this was my second F2, my first one was in superb condition when I bought it) but they were a little less robust than the "K Series" bikes, and even though they produce a whopping 3 BHP more than the earlier engines, and handle a little better, this is not reflected in their resale value, when I eventually sold mine, I only got about half of what it cost me back.
As nice as they are, they're definitely not an investment like an early K bike. Cheers, Terry.