Well, your inherent problems with Tomos are, of course, the screwed up bolt sizes, where the carb is located, and the fact that most dealers just slap them together.
Moped speedometer/odometers are more often than not, junk...why put a good speedo on something that really doesn't even need one - is the attitude there.
As for the hard shifting, seems the Tomos A5 system is notorious for this, probably cause they applied as little as possible clutch material on the clutches, three SMALL strips of some pretty wimpy stuff, and they cut the gears straight and cheaper than they did in the A3 Transmission.
The A5(A35) Trans was billed as an upgrade to the masses, but the truth is that it's a cheap, shoddy knockoff of the A3, which is why it is so problematic...they took a good, solid design, and rebuilt it so cheaply that it barely works.
The Transmission Casing is made of some pretty soft aluminum, and yeah, you get some strippage problems, not just there, but the head bolts you have to be REALLY careful tightening down cause of that.
What drives me nuts is those cursed off-size bolts, and some are metric and some ain't...like the allen screws holding the trans casing together are 4.2mm and as you said, the drain plug is odd sized as well...you more or less wind up having to find a close sized tool and file it - or you will strip something in a hurry.
SOP for a Tomos straight from the dealer is to tear it completely down, and put it back the way it should be, since most of em get it wrong, and while yer at it, replace those messed-up size bolts and screws with something more useful.
Work on an old A3 Bullet once, and you'll see where the "Quality" reputation of Tomos came from, but as far as these A5(A35) systems go, I am not impressed, and the Revival just doesn't do it for me.
Give me the old 1980's Tomos A3 Golden Bullet TTLX, imop the best 'ped ever made.