the hoses are screwed onto the adaptors fitted to the spool block, usually with a swivel nut so simply undo them .. can't really tell from your pic of the return pipe whether it's a solid hose tail or a swivel on an adaptor, but looks to be a solid hose tail, so need to detach the pipe or at least enable the fitting to swivel in the pipe, unless the other end is free/detached.
I see now that the cage is surrounding the coupling between the motor and the pump (keep fingers safe) .. the pump will have some form of flexi coupling between the motor and the pump (many options available),but is highly unlikely to separate easily, by simply removing the pump flange mounting bolts ... you'll almost certainly need to expose the coupling, by removing the guard mesh cover.
tank lid looks to be stuck down with some sort of sealing compound (silicon RTV/Hermetite/whatever). gently prise it off
... assume the pipe coming off that inspection cover is your oil pick up for the pump .. if so it should have some sort of strainer/filter on the bottom of it, that'll almost certainly need a good clean
... put your hand in the return filter hole and have a feel about
Ram in question looks to be pitted .. this will damage your gland and scraper seals in the ram end cap, which'll cause it to 'leak oil'
bottom of tank certainly looks to be a drain plug.
as for replacing the engine with an electric motor ... what HP is the engine? Dare say it runs at circa 3000rpm ... average leccy motor runs at 1420rpm ... you will also need a motor with a similar HP capacity as the engine you remove .. you only get out what you put in, power-wise ... will make the machine slow and gutless other wise .. a 5hp, 240V motor will be LARGE and from memory, at one time, you had to notify the leccy board if you were intending on connecting a 5hp motor to their supply (draws a LOT of current) ... and will NOT be a cheap thing to source. be better off banging a hose on the exhaust and feeding it out of a window
black oil is not good ... should be a golden yellow fluid you can see through ... bare in mind that each ram will have at least one end of them full of 'the black stuff' whether open or closed. one side of the ram piston will be harbouring what you want to remove ... if possible, stretch the dipper/boom out on the ground, detach the pin from the rod end, with the rams closed, fit a drain line to the gland/open end of the ram, detach the hose feeding the other end of the cylinder and GENTLY extend the ram with some compressed air ... make sure the drain line is well captured into, say a gallon can, as things can happen fast as the air pressure builds and forces the crud out of the other end of the cylinder ... alternately and more preferably -- take the rams off completely and carry out the process of purging them on a bench, with the ram held securely .. remember it'll happen quick if you don't have a good method of regulating the air pressure and introducing the pressure gradually .. even then it can still move pretty rapidly as the air pressure overcomes the oil's resistance. repeating the process from each end a few times'll get 98% of the crud out
Whenever I buy a new (to me / s/hand att I have to purge it, as my Hydrema runs bio-oil which does not suffer contaminants (mineral oil/etc.) happily ..
