Heat accumulation is a problem with high power LED headlights. At best, it will result in inefficient operation. At worst, it will result in premature failure.

Modern LED headlights have integral heat sinks which vent heat generated by the LED to atmosphere. The case is an integral part of the heat sink system. The airflow at the case is vital for the heat sink to work. I'm afraid that putting a modern LED front light into an old case will not be a viable solution.