Valid candidate? Anything that drives a speaker is a valid candidate. The venerable Fender Champ is a 12AX7 and a power tube. I don't know the 6CM6, but it is a pentode power tube driving a speaker.
You plan to put the chassis in something else? Or leave it in the tape deck? What speaker is in there, a 5x8 oval or something? 4x6?
The 6S4 is indeed the bias oscillator - says so right on the schematic. It makes the high frequency bias for the erase and record heads. I'd just pull that tube out and that eliminates any interference from the bias oscillator.
This amp is single ended, meaning one power tube, not push pull. There is no phase inverter in a single ended amp. There is no second power tube to need the inversion for. The two dual triode tubes are simply four gain stages in a row.
Like most tape recorders, there is a many-sectioned R/P switch - record/playback. The same amplifier stages are used for both purposes, but they need various changes when they are in one mode or another. The MIC input is nly live in RECORD mode, otherwise the playback head is connected to the input. All thos SPDT switches throughout the thing are parts of that R/P switch. You can make sure it is anchored in the REC position or remove it and solder jumpers in all the right places.
There are a lot of places along that signal path to massage the tone, and in fact they already did that because the needs of the recording process and playback processes are different. In fact in a couple places you may find it better to wire the RR/P in the "wrong" position at one switch or another.