Let's think a bit about what modifications would need to be made to the Whirlwind by putting radials onto it and the impact it would have on its timeline. If it were to be fitted with radials, it would have to be designed from scratch with them and then there is no guarantee that it would have gotten into service any sooner. The problem was not just the powerplant, but the firm itself.
Now, I can be one of those contrarians Admiral mentioned, but, like Dave and Steve (Graugeist and Stona) state, examining the facts as they stand is not being deliberately contrarian, but advising of the realism behind a particular situation.
(With that out of the way...) Let's look at Westland as a company. In the late 20s to early 30s, Westland was busy, but compared to the likes of Bristol and Shorts and Vickers, for example, it was a small company with a small workforce, although its book orders were large for the time. It's primary bread and butter was the Wapiti, of which it produced a total of 558; the most was of the Wapiti IIa, of which 430 were built between 1929 and 1932.
Until the Whirlwind, the Lysander was the most modern aircraft Westland had built. The Wapiti was essentially Great War technology - it was an open cockpit, fixed gear externally braced biplane with a metal tube fuselage covered in fabric, although the Wapiti I was all-wooden structure. The Lysander didn't offer too many more challenges to the Westland production staff when the first order for 169 aircraft was placed in December 1936 - its first flight had taken place on 15 June from Boscombe Down rather than from Yeovil following taxiing trials. The first production Lysander was received by the RAF in May 1938 (16 Squadron at Old Sarum).
Technologically, the Lysander was very much 'steady as she goes' rather than pushing the boundaries. That's where the Whirlwind comes in. Westland had never built an all-metal high performance cantilever monoplane with retractable gear and enclosed cockpit in a semi-monocoque fuselage before. The workforce had to be trained how to do this, the workshops had to be equipped to undertake metal bashing and the production line had to be modernised.
The Whirlwind prototype completion was delayed by supply of engines and undercarriage, it had the fourth and seventh production Peregrines, it first flew on 11 October 1938, after instruction to proceed in February 1937. The first production order came in January 1939 for 200 aircraft, with a promise from Petter that the first deliveries would be in nine months time, but the first production aircraft first flew on 22 May 1940. By this time the 200 order had been cancelled and in December 1939 an order for 114 was given, by the Air Ministry. Delivery of the first Whirlwinds to the RAF took place in June 1940, to 25 Sqn.
In his book Westland Aircraft since 1915 (Putnam, 1991) Derek James states that five years had elapsed between issuing of F.37/35 and introduction into service and that many were surprised when the order was given to Westland, as Bristol, Boulton Paul, Hawker and Supermarine were among those who offered submissions to the specification.
Delays to production aircraft came from Rolls-Royce, who was experiencing issues in producing Peregrines; part of the delay came from the supply of Hobson downdraught carbs.
So, this gives an idea behind timelines and changing the powerplant isn't going to offer much less time on that taken in real life to get the Whirlwind into service sooner, despite not having the delays from getting Peregrines from RR if a different engine is used.
