The mc18 and mc21 use a normal conventional key like just about every other bike in the world, the mc28 does not, it uses a credit card sided "smart card" instead which slots in under the clocks and is matched to the individual CDI.
Said card contains some of the ignition map as well as what to display on the LCD dash (the MC28 has just a rev counter and a small LCD rectangle.
Without any card at all the bike cannot be started.
Even with pipes, airbox mods etc. the standard Honda card is still a massive restriction, you need the HRC card for anything near 60 bhp.
But you'll lose the speedo, low oil warning, side stand cut out and neutral indicator as the HRC card make the display into nothing more than a big temp guage.
Here's the HRC card;
And it in it's place (just below the digital display on the left);
All of this is irrelevent if your going to choose model based purely on performance/value as you definately are not going to buy a '28.
You buy a '28 for one reason, you want a '28.
The '21 is far better value, cheaper to buy (probably 2/3rd's the price at the most), cheaper to derestrict (i.e. free instead of several hundred pounds) and definately no worse a bike in any way.
_________________
Please do not PM me technical questions, if you can't find it on the Forum start a thread