Since you have front wishbone suspension, you will get very little help from the AE86 community as AE86 has mcpherson struts which is fairly different.

You can fairly easily go to coilovers, but it's unlikely you'll find a kit for RT104, you have to put it together yourself. First measure the distance between the 2 shock mounts with the car at ride height. Then there are thousands of online shops, most of them from the USA who deal with hot-rods and custom applications.

If the car had a 18R-C engine, then a 18R-GE engine will bolt in with the same gearbox and mounts. But to be honest that engine is just too old unless your building a historic car. Going to a 3S-GE of some sort is 16Valve and a better starting point. A gearbox and engine mounts from a ST141 Corona would be a good start.

Honestly your better to search Toymods.org for this stuff.