Hello,
Welcome to the wonderful world of the T25.
There are the lucky owners of the prehistorc and rare ct engines, with the startling bhp output of two citroen 2cv engines lashed together,allowing you to thunder alond at about 95kph.
(lightly laden and with a following wind.)
In their wisdom the great engineers ensured compatibility between bell housings, but then to add to the pain, or is it humiliation,

used a 4 speed box for the CT.
The flywheel only becomes an issue if you have a really early 1 bolt crank shaft, later all 5 stud and interchangeable.
You will have no great difficulty in fitting a higher swept volume engine, but it will be wasted on a 4 speed box.
Several possibilities: Lo cost option, improve the performance of the CT engine, by this I do not mean adding twin carbs etc, but for £80 worth of machining and a bit of effort on your part, you can safely up the output by 20%.
Mid cost option:use bellhousing off CT box mated to a known good 5speed box with 2 litre engine
Hi cost option, get full hydraulic box, forget the 4 cylinder and go porsche, 2.2 litres or 2.6. Remember that petrol is not getting any cheaper,
There are always several ways of solving a problem EG: the american way...keep throwing money at it till it goes away. or (quote) Neville Shute: An engineer is someone who can do for ten bob what any fool can do for a fiver.
Spend your money not only well, but wisely.
Cordialement,
