Derek
You say you can detect play in big end?  Are you sure it isnt little end?
Big end failure is normaly caused by infrequent oil changes thereby leading to blockages in various oilways.
The only way to tell if the crank is knackered is to measure the run out with a micrometer to see if it still falls in the makers specifications limits.
If out , this will involve a crank regrind and the subsequent fitting of oversize shells  Also check for piston wear as you say the engine is noisey, very rarely are noisey engines the result of one part failure!!!
Good luck
Kev

I have just bought a TR1 with a big-end failure. When I stripped the engine, I found a lot of play in the front cylinder big-end. Further strip revealed that the crank is knackered. I am told by XV Motors that oversize shells are not available, so it is not worth a regrind. I have purchased a 2nd hand crank and rods, and am now in the process of rebuilding the motor with new shells, new main bearings and a very creful clean out of all the oilways.
I could not find any other problems with the engine so I am mystified as to what caused the failure.
Cheers
Rob