Go ahead,Why not.
Be warned though it will cost more than you think, It will also take longer that you think.(mine did anyhow).
I was happy with my little a12 datto but wanted more power.
I decided to go for an L20b motor into a 120y. It was a time consuming job for my mechanic but he will freely admit not rocket science.
The fj should be an easy fit. You will probably need a new crossmember , struts, heavy springs, gearbox, radiator, custom tailshaft, hydraulic pedal box clutch thingy, at least 13"rims (to clear the bigger brakes) and most likely a stronger diff.
Best of luck what ever you do.
Love the coupe