It's not worth messing around with the cam unless you've got the engine out and stripped. In that case you could have it re-profiled, but quite honestly, the lighter weight of the 4 compared to the 5 is more of a bonus than a hotter cam - ie the engine has less weight to pull.
Again, if the engine was out anyway and you just happened to have a 1300 Mk1 5 sitting there, pop that engine in with all the ancilliaries and mate it up with the 4 gearbox.
The 28IF downdraught carb originally fitted to the 4GTL will go straight onto the 5TL with it's manifold. You would also need to retro fit the 4's rocker cover and water pump etc. A problem would be encountered fitting the 4's air filter unit as the bonnet line on the 5 is lower than the 4. Again, cables would need to be modified.
All our (and the Mk2 5's) manifolds are in one casting anyway, so no need to waste money on hacksaw blades. Also the Mk2 5's exhaust outlet is in a different postion.
The Mk2 5 waterpump is completely different to the Mk1 5 and could not be used.
Following an interesting discussion the other day, it
may be possible to increase the 1108's capacity to 1400cc by using the liner/piston/conrod assembly from the Mk2 5GTL in the 1108 block. I would need to do some further research into this.
Saying that, the carb mod I did on my 4 pushed the top speed up to 90mph with healthy acceleration to match, so before considering spending lots of money in making your 4 the fastest one on the road, ask yourselves whether it's really worth it. Sounds like fun though
Steve