I know this wasn't asked, but in case anyone was wondering:
The TZX files work fine. You just have to switch tapes/sides with Home>Media select when needed and then use the tape controls to pause/stop as required (for the benefit of anyone yet to try this: Home+Down for tape menu).
For the TAP file... it works fine until it doesn't. The tape controls are unresponsive and unless I'm missing something, you'll have to use the following workaround:
- When it freezes on a "Insert Side" screen press Home to go to the options menu.
- Go to Media Select and press the listed button to clear the "current media"
- Close the media select and then the options menu to return to the game
- it still won't work because now there's no "tape", so head back to the options menu and into Media select
- re-select Sweet Fightin' +II and close the media select and then the options menu
- the game will now auto-load to the required fight and you can go about your brawling.
You may have to do this again during your playthrough, depending on what data the game needs, but it's still a lot quicker than using the TZX files.
The seemingly pointless step of returning to the game without a tape inserted is because even if you remove and re-add the TAP straight away, The Spectrum doesn't seem to recognise that anything has changed.