Boat has TMC toilet - no local spares. Would it be easy to install Jabsco (manual type) instead ?

You tell us. You know the TMC, you can find a Jabsco, you know what your boat fittings are like.

I decided to replace an old Lavac with an expensive new Lavac instead of a much cheaper Jabsco, because it fitted the moulded-in pedestal and all the fittings lined up. No regrets.

I can say that taking toilet fittings apart is a pain in the *** - it's messy, the pipes need warming to soften them to get them off the valves, and when it's back together it probably leaks so you have to do it over. And that's if you don't have an "ick" problem dealing with it in the first place.