Well, I'm guessing as much as you on what those older components use as their datasource and yes, I've struggled in the past to re-build "better" versions of out of the box components due to the API not covering all the data/functionality we need to actually do it. BUT, of course there is a "but", there's a pretty ugly way of doing it anyways. The gist of it is, you make an @override of the OOB widget, assign <@delegate /> to a variable, so you get the original HTML markup of the OOB widget in a variable, then you regex the s*** out of it and extract information you can then use to make API calls, usually the ID does it, e.g. in your scenario you'd regex out the user id's for the kudos leaderboard, then you create API requests for using those rendering your own leaderboard... It is of course ugly because first it loads the OOB widget (using server time for that I assume) and then you come in, regex stuff out, then make additional API calls to build your own, so I'd definitely recommend using <@liaMarkupCache /> in those kind of "solutions".
But before you do that, have a look at API v1, for example here https://devdocportal.khoros.com/t5/Community-API-v1-Reference/bd-p/restv1docs?section=commv1&leaf-id=Blog.kudos.authors.leaderboard#Blog.kudos.authors.leaderboard
Hint: just type "leader" into the searchbox to find API endpoints for various leaderboards. My guess is that a lot of older OOB widgets still use API v1, as that was the only thing that existed back in those days.
EDIT: Went digging in some old codebases because i wondered what it was I did this for, here you go (had to adapt the code slightly as it used functions etc. defined elsewhere, so it's untested, but that's the bulk of it anyways):
<#--
/**
/* There is no leaderboard API available for top solution authors (there is for kudos...)
/* and it's nearly impossible (only by querying ALL users and then their authored solutions
/* with "SELECT * FROM messages WHERE is_solution = true AND author.id = <id>") to do by ourselves.
/* So we parse the studio component widget for top solution authors and crank out the user ids listed
/* there, which we then can use to actually fetch the user objects.
/*/
-->
<#-- ids will finally store the extracted user ids -->
<#local ids = utils.sequence.builder() />
<#assign cmp>
<#assign markup>
<#-- @delegate seems to always be available, but can be empty! -->
<@delegate />
</#assign>
<#if !markup?has_content>
<#attempt>
<#assign markup>
<@component id="solutions.widget.accepted-solutions-leaderboard-taplet" />
</#assign>
<#recover>
${"including markup from @component 'solutions.widget.accepted-solutions-leaderboard-taplet' failed!"}
</#attempt>
</#if>
${markup}
</#assign>
<#-- Extract required data from original component markup, regex101.com can be your friend! -->
<#assign matches = cmp?matches("\\/user-id\\/(\\d+)") />
<#list matches as match>
<#-- Only add a match once -->
<#-- The ?groups build-in contains a sequence with [0] = the whole match and [0+groupn] items with their match -->
<#if !ids?seq_contains(match?groups[1])>
<#assign ids = ids.add(match?groups[1]) />
</#if>
</#list>
<#-- then get user objects from all extracted ids -->
<#assign query = "SELECT * FROM users WHERE id IN('" + ids.build()?join("', '") + "') />
<#assign response = rest('2.0', '/search?q=${query}') />
<#-- do someting with the data -->
<#list response.data.items as obj>
</#list>
