The really annoying thing is ever the tiny 87cc had an end fed crank
Something BSA neglectrd to do an anything other than the B series singles tha have all ball & rollers so they would have been happy with splash lubrication.
BSA eventually sold the stationary engine division to Villiers
So the Villiers 4 strokes are BSA Power Plus engines with a slightly altered cowel.
Randsome & Marles ( UK ) plain old Ransomes in the USA at some point in time boght the rights to the 98 cc & 120 cc engines and was making them branded BSA right up till the 90's fitting them to their reel mowers from 14" to 28" cut
FWIW low Hp range
Model B , 98 cc 54mm x 44.5mm 1.9 hp @ 3600 rpm
Model C 120cc 56mm x 50mm 2.25Hp @ 3600 rpm
Model D 220cc 68mm x 60mm 3.75 Hp @ 3000 rpm
Note these are all over square engines and had flywheel magnetos , mainy Wipac
High Hp engines
Models A & F 320cc 76.2mm x 69.8mm 5.5Hp @ 2750 rpm
Models E & G 420cc 82mm x 79.4mm 6.25Hp @ 2750rpm
These are as Shane descrbed with external magnetos & again over square .engine with end fed crank
A & E had shallow sumps for things that don't move , generators . compressors etc.
F & G have deep sumps for things like tillers that do move.
Final series Called Series 90
All 87 cc 54mm x 38mm 1.9 Hp @ 3,600 rpm
Appears to be 7 or more models Depending upon PTO dimensions & orientation of mounting bolts
Also one vertical shaft suggesting it was to be fitted to rotary mowers
No mention of the prior to WWII and the use of the Invictus ( horse logo ) sugests that they were a company BSA took over during or immediately post WWII .
If any one asked me I would hav said the sell off was in 1958 but the series 90 catalogues is dated 1961 and mention of the previous A-G models after 1958
The earliest mention I have of them is 1940 coupled directly to a generator for recharging aircraft batteries
In Kim"s lattest ( or previous) edition of the BSA literature is some of the Data sheets and most of the E series service sheets ( same as for A10's etc )