So, 91 Volvo, cranks, no start. I presume, based on the crank sensor replacement, you have no spark. I will also presume it a Bosch system, not the Regina Rex.
Check every fuse in the vehicle. Every. Single. One. There are a few power supply wires coming off the battery. Make sure they are all good.
Find a 12v test light, a stethoscope or long screwdriver, and a friend. Have friend crank engine while you listen for the fuel pump. Now use the stethoscope to listen to injectors while cranking. Now pull coil wire from distributor, hold it half an inch away from something metal, and have friend crank while you watch for spark. If you have none, you've already replaced the likely culprit with your crank sensor, so there's trouble a-brewing.
So- pump runs, no spark first. This means your ecu is receiving a crank signal, and knows to engage the fuel pump relay. Get your test light, and probe the positive terminal of the coil. Should be 12 volts with the key on. Probe the negative side, watch very closely, and crank. Light should flash when cranking. No flash, replace the ignition "power stage".
http://www.ebay.com/itm/One-Volvo-240-740-940-Saab-900-Bosch-0227100124-Power-Stage-Ignition-Amplifier-/391717231436?hash=item5b342a9b4c
Flash, hold the coil wire really close and see, or even feel, whether there is any spark at all. Flashing, no or almost no spark needs a new coil.
Alternately, spark, no fuel pump operation- Remove the ashtray and the little box and lighter under the radio, unclip the fusebox and pull it all out the hole.
http://www.ebay.com/itm/Volvo-1991-740-WAGON-Fuse-Relay-Board-From-Center-Dash-Great-Parts-Group-/152354578948?hash=item23790a6204:g:t4sAAOSwa~BYT~S8&vxp=mtr
The six pin relay, middle row on the left is the efi main relay. It is a two stage relay. Turn the key on, it should click and energise the injectors, maf etc. Cranking, it should power up the pump. Pull the relay, and see if its burnt the electrical connector. Then, probe for power. Big red? Pink, maybe, wire should be 12 volts. Big yellow wire is out to the pump. Jump them, and the pump should run. No run, beat the pump with a rock. If it runs with the jumper, pop the cover off and resolder the circuit board.
I just remembered, yours is new enough it will have a radio suppression relay under he hood that powers up the injectors. If you have spark, fuel pump, and no injector pulse, find that relay, which should be clipped to the strut tower, I think, possibly beside the fan relay if it has one. Resolder it. I think its the turbo cars that have a bank of resistors on the left inner fender with green wires. That is the power distribution out to the injectors. I've seen one of those come apart at the wire connector.
There. That should have you thoroughly confused.
Start by making sure about spark and pump/injector operation.
Timing belt isn't busted, right?