Good day,

Making the move to 64-bit does have the innate tenancy to put a bit of a kibosh on things - one such thing being 64-bit General MIDI plug-ins.

After some searching I came across two that might be of interest:

Sound Canvas VA
VSTSynthFont

Sound Canvas VA sells for $125.00US
VSTSynthFont 3 sells for $16.75US
Released: May 27th, 2019
I suggest using the Timbres Of Heaven GM_GS_XG_SFX V 3.4 Final soundfont.

Except for the price, the Roland plug-in is the most convenient.

You can also obtain a soundfont plug-in sforzando for free - I have not tested this plug-in yet, but coming from Plogue, this plug-in should be alright.