That shouldn't be too difficult... the tube base appears to be pin for pin compatible with a 7JP4, but one must connect the "post" connection to a positive voltage. For a "quick & dirty" it could be grounded or tied to the set's "B+", but would be brighter if a small 1-2kv module were added to the chassis. Focus and brightness controls would likely have enough range to work ok. The major difference would be the CRT deflection sensitivity which is about 3-4 times more sensitive than a 7JP4, so serious "over-scan" would occur if it was plugged into an unmodified 7JP4 set... a simple voltage divider could be added to the set's horizontal and vertical deflection circuits to remedy this.
P-31 is a pale blue-greenish-whitish color, which is suspect would be fairly watch-able, even though it may have somewhat longer persistence than P-4.
Could be the basis for *another* interesting "pre-war" replica-set.
jr