Away from line-of-sight, the humble WWI craft might prove difficult to detect. Here are some approaches that might be used by Martian invaders for fielding precision weapons against low-heat-signal, slow moving airborne targets:
Create larger, slower, more expensive missiles with more powerful sensors
This involves building an advanced, missile-mounted antique plane detection system. The martians would have to producing enough of them to field, attach them to missiles, and sacrifice them upon detonation, expending components and rare materials. Such systems probably rely on tracking engine noise, or high visibility of the Fokker in daylight. Strategies to defeat this would include flying at night-time, or switching the engine off to glide. Such a scenario would result in cat-and-mouse games, as the Martians listen for buzzing WWI bombers at night-time, hesitating to fire off their expensive missiles.
Invest in reconnaissance platforms
One high-altitude reconnaissance craft can guide several missiles. The missiles themselves can be very inexpensive, containing only propellant, steering and communications equipment. A potential drawback of this approach, in a planetary invasion scenario, would be high network load (hundreds of missiles flying at once). This might be exploited by Earthly defenders, who would employ swarm tactics to overwhelm the reconnaissance system. Fortunately, Earth planes of the WWI period fly very slowly, decreasing the need for Martian missiles to consume tracking data. The slow speed would help the Martian missiles keep on target, even with network overloading issues.
This tactic would make the reconnaissance equipment itself a target. Battles would be fought to keep the "eye in the sky" open.
Increase the missile blast radius
World War 2 anti-aircraft rounds had a blast radius of about 100 metres. The goal was merely to project shrapnel (flak) into the air and let the enemy planes fly through it. Given their expertise in high-energy thermal devices, Martians could increase the effective range of a single detonation to at least a mile. Flying at 120 mph, and with fore-knowledge that they are about to be targeted by a single round, a Fokker would have about 30 seconds to get out of range. If large-radius anti-aircraft rounds are as inexpensive to the Martians as WWII munitions were to humans, they could potentially be used in a barrage, making large areas lethal for aircraft to fly in.
Potentially, such barrages could even be guided from observation bases located on Mars itself. Using a little trajectory prediction, aided by the limited top speed of the Fokker, the Martians could likely hit their targets even with a 6 min (closest Earth-Mars round-trip time) delay for tracking.
Ignore the planes until they come into line-of-site
WWI weaponry had limited range. Approaching close enough to cause any damage, even to Martian assets with minimal defensive weapons, would likely be suicide.
One possibility is for human defenders to retrofit purloined Martian weapons onto their planes. However, this would be limited by the low carrying capacity of WWI planes. High-energy weapons tend to be very bulky (size and weight are apparently the main obstacles for DARPA's laser systems) or generate a lot of waste heat, so fielding airborne offensive platforms in meaningful numbers would likely remain impossible for Earth's defenders.
Thus, zapping humans defenders as they appear might, in itself, be a most effective Martians offensive tactic.
(This last point isn't exactly an answer to the original post, just a note that precision-guided weapons might not be necessary.)