A mongoose will cope with this power easy, so unless something has come adrift inside ?
I would certainly look at cam timing(possibly retarded), but if a worn dizzy they can be repaired, some pick up the top bush and spin it inside the housing as mine did, I had a gap of 0 - 2.3mm!!
Odd thing was the ONLY tell tale sign was a 1/2 misfire in REVERSE only,(due to revs being genrally lower I assume).
There is no way you WON'T feel this on the road, very bad it is, what is more important as a clue is what the missions were doing at this point ?
tabetha