I would recommend starting fresh with your MPs. You can use the following script to grab all the overrides in your environment. After you run the SCOM Data Collector check under the output zip file: CSV\Overrides.csv
https://aka.ms/SCOM-DataCollector
ALTERNATIVELY
You can also use the following TSQL Query to gather all the Overrides set in your SCOM Environment, run against the OperationsManager DB:
Select WorkflowType, WorkflowName, Overview.OverrideName, OverrideableParameterName, OverrideValue, OverrideDescription, OverrideEnforced, OverrideScope, TargetedInstanceName, TargetedInstancePath, ORMPName, ORMPDescription, ORMPSealed, MPTargetClass, TargetManagementPack, ModuleOverrideId, ORMPLanguage, OverrideLastModified, OverrideCreatedOn from (
SELECT 'Rule' AS 'WorkflowType',
rv.displayname AS WorkflowName,
OverrideName,
op.OverrideableParameterName,
mo.value AS OverrideValue,
lt.ltvalue AS OverrideDescription,
mo.enforced AS OverrideEnforced,
mt.typename AS OverrideScope,
bme.displayname AS TargetedInstanceName,
bme.path AS TargetedInstancePath,
mpv.displayname AS ORMPName,
mpv.description AS ORMPDescription,
mpv.sealed AS ORMPSealed,
mpv.LanguageCode AS ORMPLanguage,
mo.lastmodified AS OverrideLastModified,
mo.timeadded AS OverrideCreatedOn
--op.TimeAdded AS MPTimeCreated
FROM moduleoverride mo
INNER JOIN managementpackview mpv
ON mpv.id = mo.managementpackid
INNER JOIN ruleview rv
ON rv.id = mo.parentid
INNER JOIN managedtype mt
ON mt.managedtypeid = mo.typecontext
LEFT JOIN localizedtext lt
ON lt.ltstringid = mo.moduleoverrideid
LEFT JOIN basemanagedentity bme
ON bme.basemanagedentityid = mo.instancecontext
LEFT JOIN overrideableparameter op
ON mo.overrideableparameterid = op.overrideableparameterid
--Where (lt.LTStringType = 2 and mpv.LanguageCode = 'ENU')
--Where (mpv.Sealed = 0 and mpv.LanguageCode = 'ENU')
--Where mpv.Sealed = 0
UNION ALL
SELECT 'Monitor' AS 'WorkflowType',
mv.displayname AS WorkflowName,
OverrideName,
op.OverrideableParameterName,
mto.value AS OverrideValue,
lt.ltvalue AS OverrideDescription,
mto.enforced AS OverrideEnforced,
mt.typename AS OverrideScope,
bme.displayname AS TargetedInstanceName,
bme.path AS TargetedInstancePath,
mpv.displayname AS ORMPName,
mpv.description AS ORMPDescription,
mpv.sealed AS ORMPSealed,
mpv.LanguageCode AS ORMPLanguage,
mto.lastmodified AS OverrideLastModified,
mto.timeadded AS OverrideCreatedOn
--mpv.TimeCreated AS MPTimeCreated
FROM monitoroverride mto
INNER JOIN managementpackview mpv
ON mpv.id = mto.managementpackid
INNER JOIN monitorview mv
ON mv.id = mto.monitorid
INNER JOIN managedtype mt
ON mt.managedtypeid = mto.typecontext
LEFT JOIN localizedtext lt
ON lt.ltstringid = mto.monitoroverrideid
LEFT JOIN basemanagedentity bme
ON bme.basemanagedentityid = mto.instancecontext
LEFT JOIN overrideableparameter op
ON mto.overrideableparameterid = op.overrideableparameterid
--Where (lt.LTStringType = 2 and mpv.LanguageCode = 'ENU')
--Where (mpv.Sealed = 0 and mpv.LanguageCode = 'ENU')
--Where mpv.Sealed = 0
UNION ALL
SELECT 'Discovery' AS 'WorkflowType',
dv.displayname AS WorkflowName,
OverrideName,
op.OverrideableParameterName,
mo.value AS OverrideValue,
lt.ltvalue AS OverrideDescription,
mo.enforced AS OverrideEnforced,
mt.typename AS OverrideScope,
bme.displayname AS TargetedInstanceName,
bme.path AS TargetedInstancePath,
mpv.displayname AS ORMPName,
mpv.description AS ORMPDescription,
mpv.sealed AS ORMPSealed,
mpv.LanguageCode AS ORMPLanguage,
mo.lastmodified AS OverrideLastModified,
mo.timeadded AS OverrideCreatedOn
--mpv.TimeCreated AS MPTimeCreated
FROM moduleoverride mo
INNER JOIN managementpackview mpv
ON mpv.id = mo.managementpackid
INNER JOIN discoveryview dv
ON dv.id = mo.parentid
INNER JOIN managedtype mt
ON mt.managedtypeid = mo.typecontext
LEFT JOIN localizedtext lt
ON lt.ltstringid = mo.moduleoverrideid
LEFT JOIN basemanagedentity bme
ON bme.basemanagedentityid = mo.instancecontext
LEFT JOIN overrideableparameter op
ON mo.overrideableparameterid = op.overrideableparameterid
--Where (lt.LTStringType = 2 and mpv.LanguageCode = 'ENU')
--Where (mpv.Sealed = 0 and mpv.LanguageCode = 'ENU')
)Overview
LEFT JOIN (
SELECT mo.ModuleOverrideId, mo.OverrideName, mpv.DisplayName as 'MPTargetClass', mpv.FriendlyName as [TargetManagementPack] FROM ModuleOverride mo
INNER JOIN Managedtype mt on mt.ManagedTypeId = mo.TypeContext
INNER JOIN ManagementPackView mpv on mpv.ID = mt.ManagementPackId
Where mpv.LanguageCode = 'ENU'
) OverridesOverview ON OverridesOverview.OverrideName = Overview.OverrideName
ORDER BY OverrideLastModified DESC

Doing this will keep you from adding back in potential issues from your SCOM 2012 R2 environment into the SCOM 2019 environment.
If you are doing a side-by-side migration, you should check out this post on side-by-side migrations:
https://learn.microsoft.com/en-us/answers/questions/167044/whats-the-best-practice-to-migrate-scom-2012-to-sc.html