Download the app "Sworkit" and do 10/15 minutes of straight ab work everyday. (You can pick which ones you want to do and then it times you for each one). I've found leg raises, 6 inch holds, abdominal crunches, full body sit-ups with a 10/15 lb medicine ball, and incline sit-ups with a plate really make a difference in core strength when riding or jumping bigger.
No stirrup work and riding in two point will definitely help you build muscle. Outside of riding, planks and basically any variation of sit-ups/crunches (there are a LOT out there) will help build core strength.