Just a few notes for you- the DMR2 has the size of a steam weevil swarm at 5'x5'x5' and the standard 3.5e swarm is 10'x10'x10', so the below example is the equivalent of 4 OD&D swarms. I have tried to keep damages in mind with that fact. Hope this works for you.
Steam Weevil (Swarm)
by Jamie BatyDMR2 102
Diminutive Vermin (Swarm)
Hit Dice: 8d8 -8 (28 hp)
Initiative: +7 (+3 Dex, +4 Improved Initiative)
Speed: 10 ft. (2 squares), fly 60 ft. (average)
Armour Class: 17 (+4 size, +3 Dex), touch 17, flat-footed 14
Base Attack/Grapple: +6/-
Attack: Swarm (1d4 plus Intense Heat)
Full Attack: Swarm (1d4 plus Intense Heat)
Space/Reach: 10 ft. /0 ft.
Special Attacks: Distraction, intense heat
Special Qualities: Darkvision 60ft., fire immunity, killer cold, Swarm Traits
Saves: Fort +5, Ref +5, Will +2
Abilities: Str 2, Dex 16, Con 8, Int -, Wis 10, Cha 2
Skills: Listen +10, Spot +10, Survival +6
Feats: Alertness, Endurance, Improved Initiative
Environment: Warm underground
Organisation: Solitary, Pair, or Brood (2-5 swarms)
Challenge Rating: 4
Treasure: None
Alignment: Always neutral
Advancement: None
Level Adjustment: -
Steam weevils are tiny flying insects that thrive on the intense heat and pressure in subterranean areas of geothermal activity. They have developed a way to turn the heat into useable energy, and many generations of steam weevils live their lives out without ever leaving the deep caverns of their home. Steam weevils glow in darkness or dim light, because of the intense heat of their bodies.
Occasionally, because of volcanic or other seismic activity, a swarm of steam weevils may be carried to the outside world in a blast of steam or lava. In this case, the swarm will be extremely agitated and dangerous to anything around it.
Steam weevil swarms are not nearly as overpopulated as most swarms, averaging about 500 individuals per swarm (rather the normal 5,000).
Combat
Steam weevil swarms are aggressive and will quickly swarm over any nearby creatures.
Intense Heat (Ex): Any creature damaged by the swarm also suffer 4d4 points of fire damage. The damage from this heat is reduced by 1d4 hp each time the swarm is hit by water or a water-based attack that round (a minimum of 1 fire damage is inflicted).
Fire Immunity (Ex): The steam weevil swarm is immune to all effects or spells dealing fire damage, whether normal or magical.
Killer Cold (Ex): Steam weevils require extreme heat to survive. Any swarm outside of a volcanic environment will only survive for 1d4 hours. In extreme cold, the swarm will survive 10-30 minutes.
Swarm Subtype: A gridbug swarm has the following traits:
• A swarm has no clear front or back and no discernible anatomy, so it is not subject to critical hits or flanking.
• The swarm is immune to all normal weapon damage.
• Reducing a swarm to 0 hit points or lower causes it to break up, though damage taken until that point does not degrade its ability to attack or resist attack.
• Swarms are never staggered or reduced to a dying state by damage.
• Swarms cannot be tripped, grappled, or bull rushed, and they cannot grapple an opponent.
• A swarm is immune to any spell or effect that targets a specific number of creatures (including single-target spells such as disintegrate), with the exception of mind-affecting effects (charms, compulsions, phantasms, patterns, and morale effects). A swarm takes half again as much damage (+50%) from spells or effects that affect an area, such as splash weapons and many evocation spells.
• Swarms are susceptible to high winds such as that created by a gust of wind spell. For purposes of determining the effects of wind on a swarm, treat the swarm as a creature of the same size as its constituent creatures.
• A swarm rendered unconscious by means of nonlethal damage becomes disorganised and dispersed, and does not reform until its hit points exceed its nonlethal damage.
Swarm Attack : A steam weevil swarm automatically damages any creature within squares occupied by the swarm.
Distraction (Ex): Any living creature vulnerable to a swarm's damage that begins its turn with a swarm in its square is nauseated for 1 round; a Fortitude save (DC 10) negates the effect. Spellcasting or concentrating on spells within the area of a swarm requires a Concentration check (DC 20 + spell level). Using skills that involve patience and concentration requires a DC 20 Concentration check.
Vermin Traits: Vermin are mindless - they have no intelligence score and are immune to all mind-affecting effects. They have no sense of morality and are always neutral. They have 60ft. darkvision and any treasure they possess (which is usually none) consists of the property of former victims.