I think it can be done.
you will need the 3sgte engine + 4wd gearbox from the celica.
Make the engine mounts, cut chassis for gearbox clearance at the left wheel.
Use a rx7 brake master cilinder to clear intake manifold.
Use the celica shifter and shifter cables. And run the celica speedo cable to the car but notice that you will have to replace the ep91 speedometer with the gt4 speedometer.
Make a custom radiator that will sit before the front support bar to clear the turbo and exhaust manifold.
Then drop the engine in at right position.....this means with the 4wd transfercase centered .
When this is done you will have to mate the 3sgte wiring with the starlet wiring.......in the mean time you will need to make some front driveshafts consisting of 3sgte gearbox side and a starlet hub entry side. You can add some spacers for the wheel clearance.
Engine is in and runs well then you will have to fit the 3sgte center proppeller shaft that will need to be shortened and attached to the chassis using welding and rubbermounts. This can only be done after u have taken out the fuel tank,...weld plates to get rid of the big hole. and fit a aluminium fuel tank in the trunk while using a walbro fuel pump upgrade.
Then you go for the most difficult part.
You will need complete gt4 rear differential assembly including subframe, brakes and torque bars.
you will need to measure this very accurate and centered. then you will know where to weld the extra support bars to attach the subframe.
When subframe is attached and you have make extra mounts to fit the torque bars you will notice that the wheel hubs are 12cm outside the ep91 at each side. This cannot be a big problem but alot off work.
You can start correcting this by dismantling te celica rear suspension ( or coilovers) and replace the rear upper mount with the ep91 uppermount. This will allow you to attach the celica coilovers to the starlet while it still can be attached to the celica brake setup.
When both coilovers are hanging you will have a base wich you can use to measure on your ep91 rear axle......to know how much you will need to shorten the 3sgte rear axles, supportbars, and torque bars.
After this is done, the wheels will sit inside the rear arches again and you will have the celica rear diff connected to the propeller shaft. You can run the ebrake cables to the e-brake and will be braking with the gt4 rear brake setup. Must use the propertional valve and abs/system sensors from the celica too.
I think there are more things that will have to be done but if you have done all this you can do the little thing that i may have forgotten too.
After all this has been done and runs well without any problem you can take you time to upgrade the chassis with some nice stability bars or a rollcage and off coarse big brake setup.
and you will need to take out you engine and rebuid it with right parts and let the fun begin
This is what i think that can be done and i think it will be very expensive...even if you do everything. but it could be a nice project.:rockon: