A healthy B18C1 motor (out of a GSR) will fetch a lot of money on the open market.
So if this motor is free, it must be in horrid shape.

The '94 motor will bolt in, but you'll need to strip it down to the block and head, re-using your distributor, intake & exhaust manifolds, evaporative emissions equipment and engine wiring harness. Your transmission will also bolt up to the '94 motor.

But before you do any of this, have the older motor tested for leakdown.
It's the fastest way to get a sense of how worn the piston rings are and whether the motor is need of a rebuild as well.

There is one fly in the ointment, however.
If you live in a state that follows California smog law, you cannot legally install a '94 motor in a '99 model year car, even of you keep the modern emissions equip.

If that isn't an issue, go for it.