I've recently came across a rotary engine with about 40kish on it since last rebuild. However, it may have a coolant seal problem.

Is it necessary to replace the hard seals, or can I just go in and replace the soft ones without issue?

 

You don't necessarily need to replace the apex seals, but if it has coolant damage, you could also be looking at replacing the housings and rotors.

 

And while you have the engine apart, you may as well do a full rebuild.

 

just out or curiosity how long do the stock seals last, water seals i mean ?

 

With the motor apart, I would at the very least measure all the hard seals per Haynes manual/FSM and replace what needs to be. (but if one apex seal is out of spec, replace them all).