townsend

why not swap engine bell housing flywheel and rear oil seal from a 300 engine onto your 200 and change your engine mounts to move your engine forward into the position of a 300. heard of this done before.

could be fuel related. when overfueling diesel burns off in exhaust and emits sparks. check injectors and fuel related. seen this before in many cars especially when blocked cat or exhasust system.

im not 100% but i can remember fitting a 2.5 n/a to a disco 200tdi. it wasnt as powerful but fitted maybe a tdi starter will fit. if your having trouble starting from cols check you heaterplugs are working as this would make a huge difference.

you will need a puller ive got the l/r special tools but difflocks kits are good but cheaper ones for around £50 can be found on ebay with a puller for damper and crank gear along with fuel pump timing pins and crank timing pins. i have managed to remove the gear with prybars before but can be very difficult.

you cant mig, gas or arc weld it. the crank is case hardened steel. as you are attempting to weld different metal types would sugest tig as this is the hottest and most controllable type of welding as you can concentrate on the crank more than the pulley or woodruff key allowing it to penetrate enough. i have welded similar metal types on pto gear to some success but not a engine crank. i would suggest replacing the crank it would be fairly cheap buy a used crank then get appropriate new mains and new big ends a few gaskets whould be about 100 + used crank which shouldnt be much more than another 100.