Which model of Telstar do you have ? AR/AS or AT/AV ?
The G4EL will not go easily into the AR/AS due to gearbox mountings being different as well as the reason you mention. There is a version of the box (G4HL) that isn't electronic but they'd be very hard to get I expect.
Re fitting a reconditioned 3sp is going to be easiest and cheapest. Otherwise converting to Manual is probably the way to go. Bear in mind that the laser uses a different bell pattern so the gearbox wont bolt up to the FE engine in the Telstar.
You can get more info over at
www.freeforum101.com/tx5