I would think that the 75 watt speaker would be a good replacement since the amp is rated at 75 watts RMS and suspected that both speakers are wired in parallel. A 12 inch is not going to line up with your bolt pattern and you would have to remove them....not to mention taking off the grill cloth.....that's a lot of work. Also you would have to cut the speaker baffle to accommodate the 12 inch. Not familiar with your particular amp but since its a K-150, I think that speaker would work for you. The 100 watt would probably work also, but seems like over kill since you would never be running the amp at full power anyway.
If you were replacing both speakers, then I would probably opt for the 100 watt ones......I like to have a safety net just in case. One of the other guys here can probably tell you exactly what wattage the original speakers were.