You could have a 45-70 barrel punched out to 45/90 if you want a little more zing.
Before you go through the trouble, you might want to shoot it as a 45-70 to check the recoil level. I had a 45-70 Encore with a 24" barrel with a synthetic stocks that will pound you when fed heavy 45-70 loads. Seven shots was all it took to break the synthetic forend and pull the screws out of the barrel. The nice folks at T/C replaced the barrel and forend, and I added some weight to rig by going to walnut stocks and swore off using Ruger #1 loads in it. Bottom line, a 6lb. 45-70 with heavy loads makes my 8.5 lb 375 H&H seem like a pop gun.
John in Oregon