Terry wrote:

Briggs and Strattons generally very sturdy and take abuse and
even modification well! Snip ...............

Then: The Natural Philosopher wrote:

I think I was quoted around £130 for a B&S 3.5bhp motor a year or tow ago..

Just the motor or complete mower?
Prices I listed were for a mower; usually the bottom of the line
model has a red base with a black engine. You buy it, stick in
the vehicle, bring it home, read the instructions add/check the
oil, put in gas (petrol) and away you go.
For the last few years it has been mandatory for manufacturers to
provide them with the safety shut off when you release the
handle.
Recently I fixed one for a friend that had hit a rock/stone which
had bent the grass guide underneath so badly it was mangled and
bent. The blade had come up solid and jammed against it. I feared
the worst for the engine!
Used a cold chisel to cut out the spot welds that held the curved
metal grass guide and threw it away. Mower works fine.
Cheers.