I have a 90 fitted with a 200tdi engine that is knackered, i also have a disco with a 300tdi which is an mot failure, will the 300tdi engine go straight in the defender using existing mounts. Also will the 300tdi be compatible with my existing gearbox or should i fit the r380 from the disco?.

engine mts are in different position, R380 box will go in with mods - needs the remote gearstick assembley from an R380 fender box / gearbox x member / engine then sits 8" further forward.

not sure about the gearbox mate but you will need new engine mounts to fit a 300tdi, you can do it yourself or there are kits available on ebay that come with the template for fitting the new mounts.

you cant swap gear lever top without splitting box and swapping selector shaft ,i would convert gear box use disco front prop change rear for 300tdi 90 shaft fit disco cross member change floor plates tunnel etc weld on mounts fit engine 300 tdi rad etc then same as standard 300 tdi for water pipes exhaust etc,but i think most people just bolt up to gear box as is and fit new mounts and work round the rest not all bolts on g/box will lign up but you wont need all off them any way