It can mount the weapons, but it has a fatal flaw:
IS tech is too heavy and bulky.
4xASRM6 on a Scorch costs 10 tons and 8 slots. On a Fafnir 4xASRM6 costs
16 tons and 12 slots. 2xLB20X? 24 tons and 18 slots on the Scorch; 28 tons and 22 slots on the Faf. That's 10 of the 15 tons between the two 'Mechs accounted for already. But wait, it gets worse! The Scorch can carry its LB20X in its arms, because they're only 9 slots each. That means the Scorch gets to save even more tonnage by taking XL, which thanks to a less bulky Clan design, doesn't even die to side torso loss. Carrying dual LB20X on
anything IS mandates a STD engine.
It doesn't end there, though- since IS Endo and Ferro are
also bulkier than their Clan counterparts, you can't get either on the 'Mech with those weapons, ammo, and an engine. The best you can take is Light Ferro, which saves you all of about one ton. The Scorch has room for Endo structure, for ~4 tons in savings. Oh, and let's not forget the free CASE that the Scorch gets
everywhere by virtue of being a Clan 'Mech. The Fafnir gets nothing from CASE at all because the only two locations it could even mount it in wouldn't have space left for ammo after mounting it.
With the same weapons and the same ammo load (we're going to assume two tons for each LB20X and one ton for each ASRM6, for a combined total of 8 tons of ammo), the Scorch actually has one
more ton to put into its engine and any sinks you may want to add. The Fafnir will require some armor shaving to even fit a 275 STD engine, with no room for sinks. The Scorch will be able to run anywhere from a 300 XL to a 350 XL, depending on how many sinks you want- it runs ~20kph faster or more. In order to successfully brawl with an assault 'Mech, you
need to be faster than average, in order to engage on your own terms. The Scorch can do that; the Fafnir cannot- not if it's going to top out in the sub-50kph range even with speed tweak.
And let's not even talk about what happens when you add the 90-ton MCII-2 to this comparison. Even with all its bonus armor, the Fafnir is not in the same league even though it can technically carry the same weapons.
Quick'n'dirty smurfy's of
both configs.