With my first R4 (GTL 1983) I drove 485.000 km with normal service intervals (90.000 km per year) without any problem.
Only the timing chain needs to be replaced after 500.000 km and maybe piston rings replacement but anyway after 500.000 km a small engine overhaul is to be expected.
Cleon engines are extremely bullet proof uhhh that is to say it won't stand .50 caliber rounds
845cc : one of mine covered 155000 miles (248000 Kms.) without problems. It ended up needing new piston rings but with hindsight that might have been due to my using a slightly too light grade of oil (15/40 rather than 20/50).
Timing chains last for ever so long as the oil is changed regularly, and with the 845s even if the timing chain breaks no damage is done to engine or camshaft as the engine is "long stroke".