I'm not sure what the problem is here. I've had / have a number of CUBS with various size engines. It is not uncommon to have to move the firewall to accommodate modern engines in an airplane like the SIG CUB as it is an older design from 40 years ago. Times and engines change over the years. The CUB cowl is actually quite large and will accommodate a wide variety of engine sizes but in many cases, YES, you will have to move the firewall back. You may have to make a new firewall to fit the new location. Not difficult ..... just lay the engine you plan to use on the plan sheet in its desired location. Project the back of the motor mount to the plans and that is your new firewall location. As far as cowl cutting, it is minimal with any of the rear intake engines. My last CUB has a DLE35 and the only cowl cutting is to allow for cooling airflow, exhaust exit and the very top front of the cylinder head and spark plug. No cutting on the cowl sides at all, just bottom, most of which doesn't show at all on the ground. Engines like the DLE 30 with a wrap around Pitts muffler easily fit completely within the cowl with only the plug and exhaust stacks sticking below the cowl line. A DLE20 with Pitts muffler fits even better. I flew a Hanger 9 100" CUB for years with a DLE30 and it easily fit in the under 1/4 size cowl. Be a modeler .... adapt and overcome! You can change the airframe structure any way you have to. You will have to extend the sheeting on the front of the fuselage to the same length shown on the plans so it will overhang the new firewall location which is very common on many airplanes. My old Hanger 9 CUB and Great Planes CUB were both done this way. Oh yes, I mention DLE engines only because I know how they fit. Any similar engines will fit the same. Side intake engines like Zenoah will require a cutout in the side of the cowl for carb air intake and perhaps carb clearance.
On some later SIG kits like the Spacewalker, they included plan drawings that showed multiple engine types and firewall locations. They stated the firewall location would depend on the engine used. Too bad they didn't do the same with the CUB. As popular as the CUB is, it would be nice if the whole kit was updated for more modern construction techniques, engines, etc. and in general, bring it into this century.
For the guy who doesn't like big engines, I like the newer, light weight 20 -35 engines in a CUB because they give you options. You can use a scale like and sized prop that smaller engines don't allow and the larger engine balances the plane nicely.You have plenty of power to get out of trouble or to just play around with but you can also throttle back to just above idle and cruise around scale like when you want to. You don't have to use the power but it is there when you need or want it. You don't have these options with a too small engine. Of course if you are a person that treats the throttle stick like it is an on - off toggle switch, these may not be for you! In years past, I've seen way too many underpowered CUBs get into trouble, stall and crash when a bit more power in a critical situation would have saved the plane. Just like the full scale CUB that seems so innocent, beyond ground loops, the stall / spin / crash is the most common destroyer of airframes.