An easy way to change RM number crunching is to divide everything by 5 and use a d20 instead. There would be less diversity, but still enough.
Example: instead of using your 147 OB to attack that orc with 35 DB with a shield and rolling a 86 attack roll (147+86-35), you would use 29 OB with the orcs 7 DB and then you roll 17 on your d20 (29+17-7). Smaller numbers. Crits would not necessarily need to be divided by 5, but crit tables are divided in to sections of 5 already (mostly).
Fumbles etc would be more difficult I think, maybe if you have a broadsword you fumble on 1-3 on a d100 roll, you would fumble with 1 on a d20 AND then roll again under 12 on a d20.
