Need to make a systemd to start (replace) init.d script thats starts a mono executable

system is a ubuntu 20.04 currently on a arch rpi4, ideally this solution will work on any systemd arch or not. I understand there are 2 ways of approaching this 1 is to replace the init.d totally, the other is to make it compliant. I would like both assuming i Understood this correctly. Reasoning here is that i currently use monit. I wish to keep that but this maybe not always so, in which case it would be more efficient to use the systemd pure solution. In either case mono produces a file with the pid entry systemd should recognise this and not throw any error or warning, as this file is used further into the application. A warning or error at this point, maybe results in a unwanted feature later.