I can't find any information about D5Y1 in Japanese sources.
The Experimental 99 shiki type 23 Training Bomber Myojo-kai (D3Y2-K) was a modified version of 99 shiki Taining Bomber type 22 Myojo (D3Y1-K).
D3Y2-K had retractable undercarriage and Kinsei type 62 engine, only planned.
D3Y1-K had a Kinsei type 54 enigne.
Source : ENCYCLOPEDIA OF JAPANESE AIRCRAFT VOL.2 AICHI KUGISHO CONTENTS
Just to clarify so that we are all on the "same page":
The D3Y1-K Myojo was a 2 seat wooden dive-bomber trainer. The idea was to use wood to save on aluminum. It had a 1,300 hp (970 kW) Mitsubishi Kinsei 54 engine. There were 2 prototypes and 3 production aircraft built.
The D3Y2-K Myojo was a single-seat special attack version of the D3Y1-K, powered by a 1,560 hp (1,160 kW) Kinsei 62. It could carry 1 800 kg bomb. The undercarriage would be jettisoned on take off. The prototype had begun construction but was incomplete by the time the war ended.
The D5Y1 Myojo Kai would have been the production designation of the D3Y2-K (Navy Special Attacker Myojo Kai).
99-shiki training bomber "Myojo" (D3Y1-K)
Myojo was planned as wooden version of 99-shiki carrier bomber.
Production as a carrier bomber wasn't achieved consequently and it was used as a training bomber. The tentative name 99-shiki training bomber type 22 (D3Y1-K) was given to Myojo. Myojo was manufactured by Matsushita airplane, and 7 planes have been completed by the end of the war.
Myojo increased 700kg compared with 99-shiki carrier bomber type 11 of the prototype.
It wasn't inferior to a prototype in a speed , but the ascent performance decreased drastically.
Myojo-kai which reduced the weight, reduced the wing area and replaced a engine to Kinsei type62, had retractable undercarriage was planned as performance improvement type(D3Y2-K), but it has ended only by a plan.