Maybe it is easier to buy another VT220 for $5.
Typical costs for a flyback for a small monochrome terminal range
from $35 (used pull) to $60 (from a TV-parts
distributor).
If you find another VT220 with a good flyback but a weak CRT it's
fairly easy to swap CRT's or flybacks around in the "take two bad
VT220's make one good one" tradition.
Tim.