I have yet to see a handbook for the D28/D29 models. Just the catalog specifications, like Leon posted. As Alan said, the D28/D29 engine was based on the 350EW, with some changes to reflect the increased power.
On the EW the drive side main shaft is drilled hollow. On the K/M32 (the 1932 version of the D28) they certainly did not drill the main shaft, so that it was a little stronger. This practice may have started as early as D28 (and the colonial 350 version of the EW not built to a weight limit), but I am not certain.
Initially, judging by catalog images, they intended to feed oil into the end of the crankshaft via a timing cover mounted oil quill. But it may have been determined this weaken the crankshaft too much. Some machines still have the boss for this quill on the timing cover, un-drilled. Most have a smooth faced cover. The oil exited the timing cover and went to a one-way valve at the base of the front cylinder, and lubrication was solely by splash.
The oil pump mounting screws and oil ports are located a little differently on the timing cover of the D28 versus the EW350, so the pump must have been a little different too. However I have not seen a D28 oil pump, just the timing cover casting.
-Doug