The only reason to use mushroom lifters, beyond some kind of restoration of a race car, would be to allow more extreme lifter acceleration, to get more area under the lift curve. However, I would think one could get a very similar acceleration rate using a hydraulic or solid roller. Add to this, one needs a cam grind specifically designed to make use of the mushroom lifter's capabilities. Any mushroom cam lobe is at least 30 years old and there has been a lot of improvement to cam lobes during that time.

But to answer your question, I do not believe anyone is making them and yes they would work in pretty much all blocks if the bottom of the bores were spotfaced if necessary for clearance.

R.