Being able to configure the fonts within a template is possible, but the template must expose a way to do that. From my brief look at may, this would be the only way to achieve what you want:
#import "@preview/may:0.1.2": *
#show: _may.with(
font: /*your font*/,
code-font: /*your code font*/,
)
Functions starting with _ are typically only intended to be used locally (within the template in this case). So this may be an unintended way of using the template.
Looking at the code, there are two lines that define what fonts will be used (for the may-sans variant):
may.typ:198:
#show: may-elements.with(code-font: "Maple Mono")
font: ("Libertinus Sans", "LXGW Wenkai"),
You mentioned that you’ve installed some of them. As far as I can tell, if you’ve installed these three, then it should work fine.