externally the blocks are nearly identical. You can bolt in the sr20 with no mods, but you will need to fit the loom and computer, and also fuel pump from the sr20det. You can run nearly any nissan fuel pump, you just have to open the top of the fuel tank and pull it out and swap it over.
To fit a u13 sr engine and box to a u12, you will need to widen the bracket that holds the mount on the timing chain end of the motor as the sr20 mount is slightly wider than the ca/ka mount. It can be done with a hammer, you need about 10mm extra width.
You should be able to use the u13 rear mount onto the u12 crossmember, and the u12 gearbox mount onto the u13 gearbox as all the u12 and u13 manual boxes are the same