"new carb, expansion pipe, gaskets, intake, etc. with a SBP shift kit" - none of these will in any way increase the "risk of a shorter engine life" & in fact could prolong it if installed properly - the only "risk" is that due to the increase in preformance you may be tempted to abuse the motor more.
Adding an expansion chamber is the single most significant "upgrade" available for these motors, as it is an intrinsic part of a two stroke system - a two smoker will run w/o one ofc, but poorly in comparison. A new carb isn't nearly as significant a change if the one you have functions properly w/the correct mixture. Upgrading gaskets to ones that don't leak is a commonplace "modification" (repair) & "intake etc." is best achieved through no more than gasket & port matching, another minor "repair" that is required due to the lack of quality control.
I would like to point out that "shift kits" do not give you any more speed than what could be achieved w/o one, provided only that the final gear ratios are the same - the difference being that in that instance the single speed would need assistance during initial acceleration as it'd be geared tall.
If anything shift kits marginally reduce potential maximum top speed through added friction/drag - the additional chains, bearings & sprockets each taking a small toll.
The application of a shift kit improves acceleration and gives you the option of reducing cruise RPM, both valid reasons for installing one & the latter may well increase engine life - if you're willing to deal with the additional maintenance a shift kit requires.
Actual stock speeds vary wildly due to the random quality, but unless major modifications are attempted (reeds/moped top end/etc), realistically it's very unlikely you'll see 40mph+ even with all the mods you mentioned (flat ground), but you may get close. What you'll get is a far smoother running motor that's more powerful than stock and if not abused, should be at least as if not more reliable as you've fixed the factory flaws.