Gregor FDB-1, any chance against the Fiat CR.42?

I think it could give a good fight but since the Cr.42 is such a solid design I feel that the FBD-1 would be forced to constantly evade and wouldn't get many chances to shoot at the 42.

The 261mph achieved by the single prototype was with an old (clapped-out?) Wasp Junior engine. - They hoped that putting a new later version of the engine in would boost the performance of the prototype to close to 300mph. - In any event, the planned production version would have had a Twin Wasp, with which Gregor hoped to give a top speed to 365 mph! It was also hoped to double the armament to four .50 cals. - If that planned performance and armament could have been achieved then it is not only CR42s that would have had to look out for themselves - It could have given the Bf109E and A6M2 trouble!

The best online history of the FDB-1 is at this link.
The Last of the Biplane Fighters > Vintage Wings of Canada

I do think the designer was a bit optimistic.

The engine in the FDB-1 was rated at 700hp/2250rpm for take-off and 650hp/2200rpm at 7500ft max continuous. No military rating but with only 50rpm or so margin it doesn't seem like much. The 261mph speed was recorded thousands of feet higher. much higher than RAM effect would account for.

The proposed R-1535 SB4-G was rated at 825hp/2625rpm for take-off and 825hp/2625rpm at 7200ft military and 750hp/2550rpm Max Con at 9500ft which is certainly a healthy boost.
However the cube root law calls for about a 46% increase in power to go from 260 to 290mph, let alone 300 mph. The account makes no mention of the altitude at which 300mph was to be achieved.
If the original power plant was so worn out as to significantly affect performance the plane probably should not have been flying at all. This was neither wartime or an airfield in a war zone far from factory support.
The part about the R-1830 engines also seems a bit iffy, It is said that it was to be the same engine used in the XF4F-3 which was the two stage engine. We can assume the proposed 365mph was to be at an altitude that favored the two stage engine but we have to believe that the FDB-1 would have much less drag than the XF4F-3, We also have to believe that the addition of over 400lbs worth of engine (not counting intercoolers) and the larger propeller and military equipment would miraculously not affect the performance.

Agree. The addition of armour and self-sealing fuel tanks alone would have eaten into the expected performance. But it might have achieved Hurricane I levels of performance with regard to speed and as an interceptor would have benefitted from its excellent climbing ability to get it up into favourable positions to dive and attack. It might have found a niche for itself in '39, 40, early 1941 but I doubt it would have lent itself to much development beyond that.