Yup I get that if it is working when it comes in th door then (as an idea) it is simpler to start substuting parts.
lots of folks like the:
courui d01
The TK35 “clone”
For an easy MT-G2 upgrade this light is attractive, the link has all the details.
“New-117 (TK35 clone) mod. Turn a TK35 into a TK35UE”
Be careful about maglite mods folks have often go so far as to start cutting them down to use different batteries types (which to me is a whole additional level of “mod”).
Lots of folks start off with changing parts on a C8, or similure they are also known as lego lights. There is a guy on ebay who sells a 10x7135 pill for his c8 lights that just drops in and make the cells the weakest link in the light (assuming the spring resistance mods have been done.
Once you start looking you will find way too many options.