Need some help repairing soft spot in floor

You might want to think about a flex drive for a cordless drill and a long drill bit to remove the rot. If you have the space it could simplify removal of the rot. Both tools are available at Home Depot or Lowes. If the rot area is not too large you get structural foam from the site below fairly reasonable....

http://www.uscomposites.com/

 

The bottom line is you need to expose the core, remove what's bad, replace it, then bond the cutout back in place. Sometimes, you have to be fairly inventive to get around things. I've hung upside down inside lockers or hatches to access a tight spot to preform a repair. Welcome to the wonderful world . . .

It wouldn't be hard to use a hook, bent scraper or other device to gouge out the core. Lots of thickened epoxy and a replacement piece of core will get it done. It only has to be pretty on the outside.