Hi,
Looking at Wikipedia it appears that the real 1965 El Camino could be bought with either;
194 cu in (3.2 L) I6
230 cu in (3.8 L) I6
250 cu in (4.1 L) I6
283 cu in (4.6 L) Small-Block V8 Turbo-Fire
327 cu in (5.4 L) Small-Block V8 Turbo-Fire
396 cu in (6.5 L) Big-Block V8
The two smaller Inline 6's are also listed bu the name "Turbo-Thrift" while the two Small Block V-8s appear to be also listed by the name "Turbo-Fire".
The larger 396 cu in Big-Block V-8 isn't listed as an option unitl the 1966 model year, I'd guess that it "might" also fit, since the 1964-1967 vehicles appear to sare the same basic chassis etc.
Doing a quick internet search it looks like Revells' 1/25 scale 1968 Chevelle SS396 has this larger V-8, so you "might" be able to swap it out into your El Camino. If I am understanding correctly from this website a Chevy 396 cu in V-8 is about 28 inches wide by 30.5 inches long and 29 inches tall, which equates to about 1.12 in x 1.22 in x 1.16 in (or 28.5 mm x 31 mm x 29.5mm).
If your not too concenred about details, you could also maybe swap in any similarly sized engine from another kit (I'd suspect that maybe any 1/25 model of an Inline 6 or a V-8 engine of less than 396 cu in (full scale) would have a hopefully reasonable chance of fitting, though you may have to do a "little" work to get it in reasonably). Occassionally you can sometimes buy individual "engine" sprues or resin replacement motors off ebay for a reasonable price and sometimes you can come across a "lot" of random scale motors for sale that people have salvaged from their old builds.
Hope that helps.
Pat