The CO2 Mosins I have seen are (even with the plastic stock) very close to the real rifle. The colour of the plastic-wood is very good and while the action doesn't feel anything like the real thing (if anything it is better! The Mosins I have shot have been far from refined) it is more than a passable replica.
I too hate the 88g CO2, too big, too expensive and with too small a shot count. 2x 12G is a much better option and easily achievable on most rifles. Most rifles of this era did not have detachable mags (or they were not designed to reload by changing the mag) so I see no issue with finding somewhere else to put the CO2 other than having to include a way of charging it.
Great Deals with : Dave Dee, Dozy, Beaky, Mick & Tich