Good luck with that. I just ran across a similar problem... How to record off the air. The U.S. changed to digital last year, and I used the converter box for a while, but eventually bought a 32" Digital TV, since I was using a 19" computer monitor (long story). Anyway, With the converter box, I was able to record programs, but gave the box to my brother, when I got the new TV. Now, having a need, I'm not seeing anyway to record, doesn't seem to be any outputs, only a dozen inputs. Maybe I missed something, the manual was kind of thin, as the TV was a refurbished model ($259 for 32", later found it poorly reviewed). Works great, no problems in the 6 months I've had it.
I can't say what you want to do, is impossible, but don't think it would be as simple as swapping out the guts. They seem worlds a part, but who knows, might get lucky. Not really stuff I have a strong interest in, maybe someone else who's into such things. I am interested in this, as I'd like to use an analog recorder with my digital TV, but kind of looks like I'll need to buy a digital one.