Since this project is intended to be assembled by a variety of sources, I'm going to try and use parts which have a number of interchangeable replacements. This effort is even more critical with the on-going chip shortages.

The alternatives listed have NOT all been tested! Probably none of them have been tested! They looked ok on very quick inspection of the data sheet. You should double check these things. Obviously the footprint and pinouts will need to match, and I'll make a quick note of the most important other attribute to look out for.

"Count" is how many you'd need for an absolutely maxed out version of this project.

"Boards" is a rough indicator of which boards you'll find the part used on. ADC, Main, (USB) Link, LED Power, MIDI, Pedal. A question mark indicates the part is optional on that part.

(𝔍) next to a part number indicates that it is, or was, available in the JLCPCB SMT parts catalog. I don't distinguish between basic and extended parts. This does not, and cannot indicate actual availability for purchase at any particular time in the future.

Count	Boards	Function	Designed / tested	Footprint	Supply	Alternatives	Important
9	A M	MCU	STM32H743	TQFP-144	3.3V	STM32H743/STM32H753 in 1 and 2MB flash. All 𝔍
16	A M U	RS-485 transceiver	ISL83490 (𝔍)	SOIC-8	3.3V	MAX14789E (𝔍), ADM4857ARZ, MAX3077EESA (𝔍)	Full duplex, >=3Mbps
48	A	Quad op amp	LM324LVIPWR (𝔍)	TSSOP-14	3.3V	MCP6L04T-E/ST (𝔍), RS8754XQ (𝔍), MCP6V84-E/ST, TLV4314QPWRQ1	Low noise, low bias current
9	A M	3.3V Switching Regulator	VR05S3V3	3 SIP / TO-220	5V	MEZD71201A-F (tall!)	Minimal external circuitry
8	A	4.5V Linear Regulator		SOT-23-5	5V	LP2985AIM5X-4.5/NOPB, LP2985AIM5-4.5/NOPB (𝔍), RP130N451D-TR-FE, TCR2EF45,LM(CT (???)	Low noise
8	A	3.3V Linear Regulator		SOT-23-5	5V	TCR3UF33A NCP115ASN330T2G AP2127K-3.3TRG1 WL9003S5-33 LN1154B332MR-G LP3992-33B5F BCT2019EUKV33	Low noise
8	A	2.0V Precision Voltage Reference		SOT-23-5	5V	[email protected] LM4120IM5-2.0 REF2920	Low noise
1-9	A? M	USB ESD Protection	USBLC6-2SC6 (𝔍)	SOT-23-6	n/a	MAX3207EAUT (𝔍) (current board revs would need bodge wire!)	USB appropriate
1	M	20MHz oscillator	O 20,0-JO32-B-1V3-1-T1-LF	"4-SMD"	3.3V	OT322520MJBA4SL Q33310F70051500	Absolute and temperature stability
1	M	MicroSD card socket	114-00841-68	unique?	n/a
9	M	N-channel FET		SOT-23-3	n/a	PMV15ENEAR, DMG3414UN, KI2312, 2N7002 (𝔍)	On @ 3.3Vgs; cost
8	M	P-channel FET	SI2365EDS-T1-GE3	SOT-23-3	n/a	AP3415E, AO3400-SOT23 (𝔍)	<50mOhm Rdson @ -4.5Vgs
8	L	Single op amp	MCP6V51	MSOP-8	>=12V		Low drift
8	L	N-chan Power MOSFET	IRF610	TO-220	n/a	too many to count	Broad linear region, fully on @ <12V. Pins are GDS
17	A M L	Indicator LEDs	Various	0805	n/a		>=20mA continuous current, Pretty colors
1	L?	I2C I/O	MCP23017	SOIC-28	3.3V	Might not be any	Features
1	D	8MHz oscillator	O 8,0-JO32-B-1V3-1-T1-LF	"4-SMD"	3.3V	OT32258MJBA4SL ; Other frequencies also options	Cheap
1	D	I2C UART	SC16IS740	TSSOP-16	3.3V	Nothing?	Software compatibility with original part?
1	D	Optoisolator	TLP185(SE	SOIC-6	n/a	TLP183(TPL,E	Footprint; Consult MIDI spec
1	D	Open drain buffer	74LVC1G07GW	SC-70-5	n/a	SN74LVC1G07DBVR
1	P	I2C ADC	ADS7830	TSSOP-16	3.3V
4	D	Pedal jack	SJ-63053A	custom	n/a		Footprint only
4	M	Dual 8P8C connectors	E5908-0T0343-L	custom?			Footprint
8	A	Single 8P8C connector	54601-908WPLF	custom?			Footprint
4	D P L	NPN BJT		SOT-23-3	n/a		DC gain; low cost
8	L	Through-hole resistors	many	many	n/a		Lowest thermal coefficient you can afford; absolute tolerance irrelevant
176	A	TIA Feedback resistors	many	0805	n/a		Low noise, low thermal coefficient, <=2% absolute tolerance

If you're having trouble getting the RS-485 transceivers, you could probably use LVDS tranceivers. Note LVDS and RS485 aren't compatible, so you'd have to decide to use them for the entire project, no mixing and matching! I haven't tested it (and have no plans to do so), but there are pin-compatible LVDS transceivers like the SN75LVDS179D that you could drop in instead of the RS-485 transceiver.

Definitely some parts missing off the LED power board. The design may shift.

The ADC board has only been tested with switching regulators, like the listed VRVxxx part. A linear regulator should also work.

A linear regulator would produce less electrical noise, and more heat. If you were to write special firmware for the ADC board which drew much more current than the stock firmware, the heat may become an issue.

If you want to try and use a linear regulator for the digital +3.3VDC on the ADC board, the main characteristics of interest would be low quiescent current, and the lowest possible junction-to-ambient thermal resistance. The MCP1825S-3302E/AB claims a typical resistance of 29.4C/W, and was the lowest I found in a half-hearted search.

A final determination of what sorts of regulator you should use, and what sorts of regulators you could use here, will have to wait for "final completion" of the ADC board.