I, personally, would not install it. The author has taken the "kitchen sink"
approach and there's at least some stuff listed as included that I certainly
wouldn't install on any routine basis. 120 patches and hotfixes? Not
including any IE upgrades, DX, etc? Note that Hotfixes, by definition, are
not recommended UNLESS the specific problem they address is present in your
system.

If you decide to try it, be sure to create a FULL backup first, and be
really ready to restore the backup. Or to do a clean install.

no.
Where did you find this sp2.1a? Not from Microsoft.
Read with caution: http://www.softpedia.com/progViewOpinions/13-9-191,.html

I'll put it like this, I just re-tried [during the last several months
testing session] that particular un-official updates. Both with an
un-updated OS and with an officially updated OS [an over-install].
I would NOT recommend an over install into a well running system. If you
want to tweak areas, then spend the time looking for individual un-official
files specific to your actual issues and needs.
You may or may not like an un-updated install, and you must finalize the
updates [21a ended in 2005]. Myself, I have tried other un-official updates
which seem to have provided more, and more current fixes [such as Maximus
Decim's]. Though each brings its own potential issues with it, depending
upon your particular application configuration, devices, etc., and some
contain 'eye candy' junk which doesn't suit my own needs or wants [oh yeah,
like I really want 98 to look like XP or VISTA, ah %$&* NO!!!!.]..

A registry /restore will NOT remove the changes and may cause issues due to
file changes.
Someone recently suggested that it may have had a un-install via the
ADD/REMOVE Windows area [or that may have been one of the other un-official
updates] search this News group for that discussion. I did not check that as
it was not the reason for the testing.

http://www.msfn.org/board/Auto-Patcher-Windows-98se-English-t80800.html

http://www.msfn.org/board/Revolutions-Pack-711-Windows-98-SE-t58357.html

Not being of the computer generation, a non-techie, I am suggesting
the above website, as the primary source for updating Win98SE. I have
this software on my 98SE install, to my eye it has never run better.
To clear my conscience, I always warn "USE AT YOUR OWN RISK!!!". This
programmer is continuing to update, patch, hotfix 98SE so that the new
final release of AutoPatcher coming soon, will update 98SE to December
2007, fully 18 months after Microsoft canceled their support. Check
out the info on this forum, its just too much to explain, the
AutoPatcher install can take almost 90 minutes (requires the Win98SE
cd) with numerous computer reboots. Because I backup my "C" drive with
Norton Ghost, I took the chance installing AutoPatcher, my P3 600, 256
megs, ATI Radeon 7000 32 megs, ultra lite cable internet computer runs
great with it, just my experience. There is also another program that
comes in the AutoPatcher package, Revolutions Pack 7.11 (REALLY USE AT
YOUR OWN RISK!!!), which must be installed after AutoPatcher for
Win98SE, which changes the 98SE GUI into either XP or Vista. I chose
the Vista Aero Blue theme with Vista Wallpaper, Vista icons (requires
ATI etc that can handle 32 bit icons), this program completely
rejuvenates my computing experience. So I feel I have the best of
everything, a complete 98SE update + a graphically contemporary
looking OS GUI. Both programs have uninstallers. Again, just my
experience, perhaps I got lucky, whatever, but, as long as,
AutoPatcher for Win98SE continues ( the programmer, Soporific,
continues to work on new versions), I will happily continue to using
Win98SE. Again I warn USE AT YOUR OWN RISK.