Daijisen says the okurigana is optional in ひとふでがき【一筆書(き)】.
Probably okay to display it without okurigana here too.

〈筆／ふで／フデ〉〈書（き）／がき／ガキ〉

Google N-gram Corpus Counts
╭─ーーーー─┬────────┬───────╮
│ 筆書き　 │ 18,015 │ 78.0% │
│ 筆書　　 │  1,825 │  7.9% │ - add
│ ふで書き │  1,440 │  6.2% │ - add, sK
│ 筆がき　 │    416 │  1.8% │ - add, sK
│ フデ書き │    120 │  0.5% │
│ ふでがき │  1,255 │  5.4% │
│ フデガキ │     27 │  0.1% │
╰─ーーーー─┴────────┴───────╯

@@ -5,0 +6,11 @@
+</k_ele>
+<k_ele>
+<keb>筆書</keb>
+</k_ele>
+<k_ele>
+<keb>ふで書き</keb>
+<ke_inf>&sK;</ke_inf>
+</k_ele>
+<k_ele>
+<keb>筆がき</keb>
+<ke_inf>&sK;</ke_inf>

unidic

Not in any of the other refs, but common enough.

daijr, nikk

Google N-gram Corpus Counts
╭─ーーー─┬────────╮
│ 筆書き │ 18,015 │
╰─ーーー─┴────────╯