How does the Cg change when a lifting stab is used and how can I calculate the CG of such a plane

if it is a standard design not a canard does not make a significant change in c.g.just compute it normaly.

Lifting stab? Sounds like you're playing with an old timer design or at least a design that had it's start in the hands of someone that does play with OT models.

The application of the lifting stab as is often found in OT designs is not the element that allows moveing the CG back. Rather it's the large percentage stabilizer coupled to usually long tail moments that allows the CG to be well to the rear compared to other more conventional model designs. This factor is known as the tail volume coefficient. And larger values for this number allow more loading to be placed on the stabilizer. With this load on the stabilizer it starts to make sense to use a lifting airfoil in order to help create this lift.

Free flight models and OT radio assist models can have CG positions anywhere from the usual 30 to 35% back from the leading edge to back around 70 to 80% for the more extreme designs. Some of the "pencil bomber" designs of the 50's had such long fuselages and large stabs that the balance point was on or near the trailing edge. A few even had 10 to 15% behind the trailing edge but those models looke very odd by most normal standards.

Thanx, Iv seen them on some OT planes back home and was curious to see if it would help a small plane. I imagin it would make little differance.