With the caveat that I have never even sat in a Rallye, far less flown one: I see no reason why you would need to pull more than 3G to loop.
All the aerobatic types that I have flown (Pitts, Eagle, CAP, Extra, Tiger Moth, Stampe) all have had min and max entry speeds for a range of maneouvers in the POH. Some were even placarded in the airframe.
If it were me and the book said inside loop 80-140 units of IAS. I'd firstly check that the ASI and the book were using the same units. If one is in KPH and the other in Knots, then I'd mentally calibrate the ASI to reflect the book. On a CoA aircraft, the ASI should use the same units but I have known where this was not the case! I assume that there is an accelerometer in the a/c - if not I'd gaffer tape one somewhere safe and visible.
After ensuring the W&B were inside acceptable limits and that the airframe was secure (loose cushions, etc), my first attempt would be solo, at something like 4000'.
Dive to 120, then gently pull back to 3G, feed in right rudder as she decellerates, you'll need more back stick as the elevator is less energised at lower speed, float over the top from 10 to 2 and then recommence my pull to 3G on the downhill portion.
If it is struggling over the top, I'd make my second one at 130units and see how that improved things, then at 140. If it coped admirably at 120, then I'd start the next loop at 110, then at 100, then 90, etc.
I would also note the start height and finish height and if possible, the speed at the top whilst inverted!
You will then have a good idea of how responsive/stable the a/c is at different speeds. This will then help you on your second sortie when you want to demonstrate the loop to someone brave.
Stik