Can I convert a Sunbeam Radiant Control T-20C toaster from 120V to 230V?
It’s unlikely that you can internally convert it. That would require finding two series strings of heating coils of the same resistance, and moving them from series to parallel.  Nothing but a teardown will tell you if something like that exists.   Even so, the switch may not be rated for 230V, though that’s probably not a huge issue.
That is an absolutely exquisite toaster.   They aren’t made anymore because of incompatibilities between their operating principle and a new UL White Book requirement. As such, they should not be trashed by risky behavior with cheap “power converters” found on the Internet or at junk shops.  There’s a special hell for people who take such risks lol.
The good news is, AC frequency won’t be a problem, so you just need a reliable source of 115/120V.  A proper transformer of sufficient ampacity is a fine answer.
You can overload a heavy transformer somewhat since your usage is momentary: Heat is why we care about overloading transformers; the iron and copper have significant thermal mass and take awhile to get warm enough to matter.  Same way little car starter motors can start a big engine.
Do not use cheap “power converters” unless you very carefully watch their power output on a scope.
Proper AC voltage conversion requires a transformer.  If it’s done electronically, there are a variety of ways to do it, in a wide spectrum of accuracy &lt;-&gt; cheapness.  At extremes, cheap “triac dimmers” for lights make a very ugly AC waveform, that works for incandescent lights intended for that same voltage, but is a mess for anything else.
You really have to watch out first for the spec of the converter, and second for “cheap overseas trash” overstating its capabilities.
Diodes won’t work.
That’s another example of way too far on the “cheap” spectrum.
Without a diode, running a 115V resistive load on 230V means you are doubling the voltage, and Ohm’s Law means you are doubling the current. Watt’s Law says if you double both voltage and current, you get four times the power/heat.  Needless to say this will instantly fry the toaster.
Now, somebody gets the bright idea “use a diode; block half the AC waveform”.  Now, what did I just say?   Applying 4x the survivable heat, 1/2* of the time, let’s see... do the math...

4 x 1/2 =

Anyone, anyone, Bueller? (Hint: 2x2=4)
That’s right... 4x the heat 1/2 the time is still double the survivable heat, and will again fry the beautiful toaster.