Tony

Gordon is right the Wren is 75mm dia and the Kingtech is 76mm dia... a 1 mm difference. The Kingtech is shorter making installation potentially easier.
Both engines produce good performance and reliability and the only noticeable difference is that the Kingtech is thirstier.
They will both happily use the same exhaust tube.

The really big difference is the cost.

The Kingtech is VERY much less at $1450 plus any import duty.
The Wren 44 is £1650 plus VAT making it £1980 to UK and European buyers
For those buying at Dollar prices the £1650 converts to about $2205 plus any import costs.
In fact you can get a Kingtech 70 for less price than a Wren 44 and have a lot more power.

Dealing with the battery issue is a minor problem.

John