Consider changing the whole crankset.. then you can use another one, say a Double with a 130 bcd

that will accept a Metal chain-guard ...
typically bolting on in place of the Larger chain ring, which in the case of a Brompton
would be on the inside position, and the chainguard disc on the outer position..

I have such a setup on my Bike Friday Pocket Llama. It uses a 53t chainring,
and they have large diameter disc Guards made for them, in Aluminum..
they, as a Business, of course would sell them, as Parts.

(I took off my original B crank & BB and fitted a Schlumpf Mountain Drive..
Perhaps you can budget instead one of their speed drives , their 34t
chainring when kicked into its internal overdrive, acts as if it were the 54t sized one.. )

its suited to go fast more than climb the Alps.