Kora Explorer 90 amp- are 12ax7 tubes ok?

In most cases it is okay, as long as the ciorcuit can handle the 12AX7 which has a gain factor of 100, while the
12AT7 has a gain factor of 60. I would be certain before you make the switch. The amp may get more noise with the 12AX7.

Remember you lose 40% of the amplification factor of a 12AX7. The At7 is considered by many a bad audio tube because it was originally engineered to work in oscillators. As an input tube however it can work very well because it is a very good phase splitter. It is apparently a very versatile tube eliminating the need evidently for a separate phase. A fair number of engineers use them.
I used a variety of 12volt 9 pins in the input section of a 40wpc ultralinear amp using a very classic Mullard long tail circuit. I found some tubes just didn't sound so great but all of them worked well enough.
I have no idea what purpose it serves in a Kora . Be warned I am a technical know nothing