Mike (TR), I have no clue about what's inside phonographs. Never even had one in my hands.
The short answer to your question would be - NO, meaning the mod would be different, that's for sure. My guess is that you may have enough parts there to buld a small guitar amp, but the preamp section would need to be re-designed and that may mean that you will need maybe a few different componets and some parts. The good thing about recorder's amps is that they already designed with input preamp section and most have mic and line level inputs. Mic input works fine for guitar pickups (also can be re-designed, or modified of course). With r-t-r machine you actually can use it as is without any mods, it's just not very practical. HERE:
Check this guy out,

(nice photos , btw

)
Here's another
webcor amp "project" (without mods, and no preamp (???)), btw, that's a kind of amp you may have in your phonorgaph, or similar, but it's just a wild guess.
Now, here's a good thing about webcors - the unit may have schematics glued on the inside of the enclosure somewhere. That's what I know from r-t-r machines, so I guess maybe phonographs also have schematics inside. The painfull part is though that it is hard (or imossible) to take it off and the old paper iss falling apart. And the lettering of the print is almost microscopical

.... (you can see the original schematics that I've took off the inside the enclosure-case of MusicMan machine, it's
on the bottom photo of this chart)
Anyhow, if you have the schematics and you can get it scanned, then I may try to sort of guess how to mod the thing, but I'm not a guy really to ask about something that I personally have not done. I suck in theories

, I'm not so sucky with a hammer, though ...well, I gotta have confidence in something at least.
Open the thing up, make a photo if you can, post here. Just be careful. Dont touch contacts. You MUST make sure that electrolytic filter cups are discharded.
SAFETY ASPECTS - read here , just in case.
Drop a note, what ever you do,
/respects