With the stuff you describe I would guess that your grandpa was going to build a scale or carrier CL plane. It is/was common to connect the throttle and brakes (or tail hook) to the third line.

I am not familiar with that particular engine and I don't know of any "trainers" of that size. If you can find a local club that has a trainer, it would be ideal. Trouble with a trainer is that after you get used to the trainer's "round and round", you usually want to do more...which a trainer is usually not capable of.

Of course after you learn to fly, you could add the third control stuff and have fun with stop and goes.

Good luck.

George