For gas engines, most 1/4 scale (9 ft wingspan) Cubs fly really well on a 20CC gas engine such as a DLE 20 RA, or an RCGF Stinger 20-RE. These will turn a 16x8 or 17x6 prop and produce about 13-14 lbs. of thrust. The engine weight with muffler and electronic ignition will be between 1.75 to 2.0 pounds. Many of the J-3 Cub models have a light wing loading and will fly in a scale manner on relatively little power. For example, my Goldberg J-3 Cub weighs 7.5 pounds and has flown quite nicely on an Enya .53-4C glow engine for almost 30 years now. I think your J-3 Cub would also do just fine with a 15CC gas engine. It just would not have a great vertical capability. If you are looking for vertical performance and the airframe structure is sturdy enough, then a 20cc gasser would probably be the upper limit. If the airframe is fragile or the least bit rotted, then stay with a 15 cc gasser. A 15cc gasser with 10 pounds of thrust should be sufficient.

For glow engines, a Saito, OS, Enya, or YS 1.20 4-cycle will fly 13 pound airplanes quite nicely too. The difference in all up weight between my Enya R1.20-4C glow and my RCGF Stinger 20-RE is about 3 ounces. Once the weight of the fuel is considered with the gas engine needing a much smaller fuel tank, the total weight difference is negligible. As a rule of thumb, I have found that for mid-size glow and gas engines, a 2C gas engine will produce just slightly more horse power as the same displacement glow 4-C engine. I.e. a 20cc gas engine will make about 2.5 horse power which is about the same as a 20cc (1.20 ci) glow 4-C. The same for 15cc (.90 ci) gas and glow engines at about 2.2 hp..

"Gasser Pros" ... Gassers have a much lower fuel cost. A typical 1.20 4-C glow engine will drink 16 ounces per 12 minute flight. That is 8 flights on a $27 gallon of glow fuel,. That comes out to $3.50 per flight. A gasser uses 100% gas mixed 32:1 with Redline synthetic racing oil at about $7 per gallon and will use about 12 ounces per flight, or about $0.58 per flight. The cost of a new 20 CC gasser starts at $250, whereas the cost of a new Saito FA-1.25 -4C glow engine is about $430. If money is an issue, then the gassers will provide more bang for the buck. A properly tuned glow engine can be as reliable as a gasser. A gasser running on a properly leaned mixture produces very little oil residue to be cleaned up after a day of flying. Usually one paper towel and a light wipe or two is all that is needed, Whereas a glow engine will use more towels and Fantastic cleaner plus a bit of rubbing to remove all the oil slober. CM-6 spark plugs are cheaper than glow plugs and usually last longer.

"Gasser Cons" ... Gassers are usually designed to be mounted in the inverted position instead of the horizontal position. This may or may not be an issue for you. You will also need to install an ignition battery, a separate ignition battery on/off switch and an optically isolated engine kill switch to allow the engine to be killed from the transmitter. An optical kill stitch costs $19 and weighs under an ounce. Fuel tank stopper plugs and fuel tubing must be rated for gasoline. As you stated, a new engine mount will be needed to absorb all the vibration from a glow or gas engine.

Decisions, decisions, decisions... Good Luck!