Yeah it's definitely an Elstar. Back in the seventies when grass track racing held more importance than it does now, many of the manufacturers made their engines available to grass track framemakers in loose tie up deals. Hagon-Bultaco, Hagon Yamaha here in Australia and Elstar Maico were some of the more popular tie ins. You could buy a brand new Maico Elstar from your local Maico dealer in the UK. Maicos were also associated with Antig frames a bit later on. I have an Antig I hope to have up and running with a square barrel 250 once the other projects are out of the way.