Update, after cleaning it with sprayed hot water and small brushes:
Note: I only used up-down moves, to prevent bending.
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I use a garden weed sprayer, its less powerful than a high pressure washer yet can have its jet aimed in a small area. Additionally you can fill it with hot water which will help loosen the mud.
Try a few applications of something like Muc-off too.
Normally its best to try to remove the gunk as soon as you get home rather than wait till its totally dried in as it should be easier to dislodge.
A liberal spray of bike protector (ACF 50, or the like) will help dislodge future build up of mud.
If you notice any bent fins, you can purchase a fin comb to straighten them again. Your motor supplies store should have one or know where to get them, alternatively try online.. Just search for Radiator Fin Comb in your favourite search engine.