Kibana development - migrate/upgrade plugins from 7.3.x to 7.9.x

[tsvet (p.)]

We are still using the old legacy architecture and we can not register the visualiations. We have different kind of custom plugins (visualizations, navbar extensions and applications). With Applications - all good but with navbar extensions and visualization - no luck.
@Pierre_Gayvallet I saw a comment from @Mikhail_Shustov and would like to give it a try with the hacks.js approach -

Debugging missing plugins after updating from 7.6.2 to 7.9.2 . I know that we should use the new platform way of developing but we have many plugins and we dont have that many resources to do the migration immediately. Any hints on registering visType visualization with legacy architecture on kibana 7.9.3 will be much appreciated