The Russian Civil War and the various sub-conflicts (such as the Soviet-Polish war and Finnish Civil Wars) make for excellent gaming material but there are relatively few rule sets dealing with it specifically.
Luckily, any set of WW1 rules (such as 1916, Great War Spearhead, Over the Top, Contemptible Little Armies etc.) can be pressed into service with a few tweaks.
And if that fails you, grab some WW2 rules that you like.
I am going to make some suggestions below, mainly for larger scaled battles:
1: A bit less trenchy.
Troops certainly dug in during the RCW, however, it was not nearly as common to find the extensive trench networks of the Western front.
Part of this is the nature of the fighting: A lot of the "meeting engagements" that players tend to enjoy would be alongside rail or river supply lines, where there's not much to be gained from holding a given spot.
Additionally, given the size of the territories in question, a heavy position could usually always be outflanked.
As such, if your game system gives an increased defensive benefit to trenches (compared to other forms of "hard cover" such as a stone wall or barricade), don't allow that bonus unless defending a real fortification network (such as Petrograd or Tsaritsyn).
Trenches are simply rated the same as any other physical obstacle.
2: Crap troops can still win.
While many units fought fiercely and resolutely, civil wars tend to give us an excuse for all manner of dubious troops.
What will occasionally surprise WW1 (and even WW2) players is that these troops often did just fine storming an enemy position.
To some extent this can be answered by the equally dismal state of the opposing forces, however I think a significant factor is that there's not much to be gained from defending a particular spot to the death.
Whether you defend this particular spot of dirt or another equivalent spot 10 miles to your rear will make little difference on the steppes.
As such, if your rules offer a Morale bonus for occupying defensive positions, do not apply the bonus unless it is a vital position or your troops are Loyal.
Alternatively, lower all "in cover" Morale boosts by one step.
One of the more fun aspects is that cavalry is a vital battle field tool in the RCW.
They can even do quite well on the actual battle field, to the delight of many a frustrated WW1 horse-miniature painter.
My personal theory as to the relative battle field success of cavalry in the east is that firepower was comparatively low: A mixture of rifle types, limited ammunition as well as modest degrees of training (to be optimistic) meant that troops just couldn't put out the sorts of concentrated firepower that would doom any cavalry attack in the West.
If your rules provide a firing bonus against mounted troops, drop this bonus UNLESS the troops in question are Allied Intervention Forces.
Yes, this will apply even for machine guns.
I'd also suggest applying a morale effect for enemy cavalry near your position.
A simple penalty (or even forcing a morale check when enemy horse approach within "close" range) will work fine.
4: Who you gonna call?
Indirect artillery fire should probably be non-existent unless you're attacking a known, fortified position.
Most artillery support would be over open sights at spotted targets, even for mortars.
This incidentally allows you to ignore the most complicated part of many rule sets.
There's plenty of recorded instances of troops melting away at the first sign of trouble or even switching sides.
Unless a unit is Loyal, if you roll the worst possible score for a Morale check, the unit is removed from play.
If they are within "close" range of the enemy, give a 25% chance of changing sides (in which case they are assumed to have passed the morale check).
I hope these tricks help or maybe even get you interested in this wonderful period for gaming purposes.
Let me know your own ideas below.