Ohlookabirdie

Is it possible to convert an old 90's mtb frame to be a suitable bike for indoor "road" cycling on a turbo? if so what are things that need to be considered?

clasher

I think making the saddle height and setback the same as your road bike is the most important thing. Reach is a bit less important for me but I ended up a bit closer since I only had a short stem on hand, it hasn't been a big deal for me so far, but I sit up a lot more on the trainer than I do outdoors so it's not quite the same.

I use the same model of crank on my indoor and outdoor setups, I find my knees don't like switching between wider mountain cranks and road cranks.

I use a direct drive smart trainer and usually free-ride in zwift so I have a wide range 3x10 setup on the trainer. I also have the kickr climb so I keep the trainer difficulty at 100%. If free riding in zwift and training to ride steep hills some people find it helpful to keep the trainer difficulty at 100% but not everyone has a bike setup for doing 10-15% climbs so the trainer difficulty can help ease the grades but it doesn't help increase the resistance on the flats in zwift, so some people with MTBs find a need to increase baseline resistance but if an old MTB has a triple with a 48t big ring that should be enough for riding around zwift, I do fine in zwift with 50x12 as a high gear... depends on how fast one likes to spin I guess.

surak

You'll want to use a slick tire, the typical knobby one will be louder and possibly have less than desired grip against the drum.

caloso

Agreed. That's pretty much all you'll need to do.