The transmission you need is a Hurth 10 with a 2.1:1 ratio.
The tranmission is the weak link in the drivetrain though. The 4-108 at 51 HP @4000rpms is right on the limit for the hurth 10 you might want to install a ZF12 (ZF bought out Hurth) but you will need to move the engine forward or shorten the prop shaft, as the ZF12 has a larger casement.
The Perkins 4-108 is a awsome engine old school tech heavy and strong. However they leek oil from front and rear oil seals. If a perkins stops leeking oil then there probably isnt any left in the oil pan.