Filters-take your existing filter(s) to a truck shop. They may be able to match them. Ditto for automotive suppliers.
Engine mount-see if other engine mounts can be adapted. You could get some ideas from http://www.polyflex.com.au/
pump-depends on the pump type. I had a bad experience having a water pump rebuilt so don't trust quick rebuild
stories. My pump is still in one piece after a quickie repair. I got a new impeller by taking the old one to a sympathetic chandler.
The ring that attaches to the front of the engine with ears for starter mounting-You may find that a reasonable metal fabrication workshop can make something up for you. A few phone
calls and a cardboard dummy could help.
A marine engineer
who does machining could make adapters up for you.
Your engine could be sufficiently small and light that it is not a big hassle to take it to a repair shop for things like sleeving water pumps if that is necessary.