90/90 R18 can be installed on CG:
90/90 R18 can be installed on the CG125 without any alteration. Its the same rear wheel size as the Deluxe has.
Check your chain and sprocket:
What usually happens when you install bigger rear wheel is the it puts pressure on sprocket and chain of the bike and if they were already weak they give up easy. Do get you sprocket and chain set checked. Im sure chain has worn off.
How to Check Chain and Sprocket
You can check the chain yourself. Put the bike on double stand and put it in neutral, and rotate the rear wheel slowly. Keep a close eye on the wheel......see if its rotates smoothly or stop or slows down abruptly. If you see any abrupt stoppage or slowing down it means sprocket and chain needs to be replaced.